Discoveries and Inventions of the Nineteenth Century (part 5 of 5)

Discoveries and Inventions of the Nineteenth Century (part 5 of 5)

Under the date of May 28th, 1665, the curious gossiping diary of Samuel Pepys contains this entry: “Thence to see my Lady Pen, where my wife and I were shown a fine rarity; of fishes kept in a glass of water, that will live so for ever—and finely marked they are, being foreign.” This doubtless refers to the now well-known gold fishes, which about the time alluded to were introduced into Europe from China, where they had probably been for ages reared and kept in captivity, chiefly for the sake of ornament. Perhaps the reader may be disposed to think that, therefore, the aquarium cannot be distinctively a nineteenth century invention, nor at all a modern invention, in principle at least; but merely the “glass of water,” or the globe of gold fish on a larger scale. Such a notion would be quite incorrect, for the principles which are embodied in the modern aquarium were not recognized and applied until quite recently. Aquatic animals kept for a period in vessels in which the water is changed from time to time cannot be considered as properly forming an aquarium. The beauty and value of a well-regulated aquarium depend not merely on the opportunities it affords of studying the habits of the animals; the spectacle it presents has a far wider interest, as illustrating and confirming the conclusions of science regarding certain great principles which govern the whole animal and vegetable life of this terraqueous globe. Perhaps in the whole range of nature nothing is more wonderful than the direct interdependence of animal and vegetable life, and the exact balance between them, which preserves the composition of the atmosphere unchanged. The constituents of the atmosphere have an immediate relation to both forms of life. No animal can live without a supply of oxygen gas, which it absorbs and replaces by carbonic acid gas. The latter, on the other hand, is absorbed by plants, for these, under the influence of light, decompose the carbonic acid, returning the oxygen to the atmosphere, thus purifying the air by again fitting it for the respiration of animals.

It might be supposed that animals which live entirely beneath the surface of water are removed from the influence of atmospheric oxygen, and that they form exceptions to this law. But such is not the case, for water absorbs and holds in solution a certain quantity of air, the oxygen of which is taken up by aquatic animals. In the lower forms of animals inhabiting water, the absorption of this vital element takes place at the general surface of the body; but in the more highly organized creatures there are special organs appropriated to this purpose, of which the gills of a fish may be cited as a typical example. The giving out of carbonic acid is an action as universal in the animal world as the absorption of oxygen, and all aquatic animals tend to charge the water in which they live with this gas. Fish, or any other water animals, will soon die if they are placed in water from which all the air has previously been expelled by boiling, or by placing under the receiver of an air-pump. In this case the creature dies from want of oxygen; but it would also die, even if supplied with oxygen, were the poisonous carbonic acid emitted by itself allowed to accumulate in the liquid. In nature, this carbonic acid forms the food of aquatic plants and sea-weeds, and these restore oxygen to the water. If a bunch of watercresses be placed in a bottle filled with water, and exposed to strong sunshine, the leaves may soon be seen covered with small bubbles of gas. This gas may be collected and examined by a suitable arrangement of the bottle, and it will be found to be pure oxygen.

The merit of having first imitated the plan of nature for the preservation of aquatic animals appears to belong to Mr. Ward, the inventor of the “Wardian cases” for ferns and other plants. He, in 1841, formed in London a fresh-water aquarium, in which, for the first time, the animals were kept in a healthy condition by the compensating action of plants. Mr. Gosse, Dr. Price, and others, made experiments with marine animals and plants, about 1850. Mr. Mitchell, who was then secretary to the Zoological Society of London, saw about this time a small aquarium on the balancing principle at Dr. Bowerbank’s, and this suggested the erection of the fish-house in the Zoological Gardens, Regent’s Park. This was opened in 1853, being the first public aquarium ever constructed. The tanks remain at the present time in nearly their original condition, and this aquarium has been remarkable, not only as predecessor of the many public aquaria which have since been erected, but for having given rise to a movement in favour of aquaria as domestic establishments. The setting-up of household aquaria became almost the rage of the day, and so many books and magazine articles devoted to the subject appeared during the ten years following the establishment of the Regent’s Park aquarium, that the literature of the subject is quite considerable. Mr. Gosse showed how water for marine aquaria could be produced by adding to fresh water the solid constituents of sea-water; and, in the marine aquaria of some inland towns far distant from the sea, this artificial sea-water is the only kind used. After the establishment of the Regent’s Park aquarium, public aquaria were opened successively in Dublin, Galway, Edinburgh, Scarborough, Weymouth, the Crystal Palace, Brighton, Manchester, and Southport; and on the continent at Paris, Hamburg, Hanover, Boulogne, Havre, Brussels, Cologne, Vienna, and Naples; also in North America at San Francisco, and in other places. The general interest in public aquaria, and especially marine aquaria on the large scale, seemed to increase as the comparative failure of the domestic tanks lessened the taste for them. The causes of the failure so often attending the attempt to maintain aquaria on the small scale arise partly from the amateur naturalist’s want of exact knowledge, and the great amount of attention and care required, and partly from the inherent difficulties of the subject. An aquarium, even on the largest scale, and with every appliance that science can suggest, only represents, after all, a few of the conditions which actually exist in nature; but in small vessels, with a limited quantity of water, without the continual motion of the liquid, which belongs naturally to seas and streams, and with circumstances of light and temperature widely different from those which are obtained in nature, it is not surprising that the success of domestic aquaria should be but very partial, and that the taste for them should have declined accordingly.

Many public aquaria proved commercial failures; but we select for special description two which have been thoroughly efficient, and are remarkable for size, reputation, and successful management. The arrangements at these two institutions as regards the aëration and renewal of the water are, however, quite different. Some plan by which the same sea-water might be supplied with oxygen, and kept in a clear and pure condition, was necessary for the very existence of the inland marine aquarium at the Crystal Palace, whereas the position of Brighton made the natural sea-water more available. The success of the former method at the Crystal Palace Aquarium, under the judicious system adopted by Mr. W. A. Lloyd, the superintendent, perhaps renders this aquarium one of the most interesting, in a scientific point of view, of any yet in operation. The water here is never changed by the addition of sea-water; but fresh water is added as required, simply to supply the loss by evaporation; and any solid constituents which the animals may abstract from the water as material for their shells is replaced, so that the ordinary composition of sea-water is maintained. This is merely imitating Nature, for the evaporation from the surface of the sea is compensated by the fall of rain and the influx of rivers, the latter constantly bringing in the various salts held in solution. The following particulars regarding the Crystal Palace Aquarium are derived from Mr. Lloyd’s excellent handbook, which contains not only clear descriptions of the inhabitants of the tanks, but interesting historical notices and a well-written disquisition on the principles which should regulate the construction and management of aquaria.

THE CRYSTAL PALACE AQUARIUM.

The building was commenced in July, 1870, and was opened in August, 1871. It was designed by Mr. Driver, of Victoria Street, and presents an admirable simplicity, which entirely accords with the purpose for which it was erected. The whole available space has been occupied, and nothing has been wasted on unmeaning or fantastic embellishments. Even the decorative shams, in which ordinary painters delight, have been excluded. No part of the walls or of the woodwork is painted to look like marble, or even to imitate oak. The building, which is about 400 ft. long and 70 ft. broad, is situated at the north end of the Palace, partially occupying the site of the portion which was so unfortunately burnt down in 1866. It is but one storey high, and besides a large reservoir beneath the floor, holding 100,000 gallons of sea-water, there is a series of sixty tanks, with thick plate-glass fronts, which collectively contain 20,000 gallons of water. This water, weighing over 1,000,000 lbs., was brought from the coast and conveyed to the Palace by the Brighton Railway Company at a very moderate rate. For many weeks after the water was placed in the reservoir and tanks it was very turbid, from taking up the lime used in their construction and in that of the rockwork. In this condition it was very alkaline; but the lime was slowly precipitated by the carbonic acid of the air, the water became clear, and vegetation appeared in the tanks. The great capacity of the reservoir facilitates the cleansing of the water; for, supposing that the water in one of the tanks, holding, say, 6,000 gallons, became turbid from any cause, the water from this tank could be run off into the reservoir, where its mixture with the much larger quantity would not sensibly affect the purity of the mass, from which within half an hour the tank could again be filled.

All the tanks are constantly receiving water from the clear and cool reservoir below, in which there are no animals, so that the motion of the water in the tanks, like that of the ocean, is incessant. The water issues from the pump at a rate (indicated by a counter) of from 5,000 to 7,000 gallons per hour. The pump is worked by a steam engine of three horse-power, and the machinery requires the unremitting attention of three engineers, who succeed each other by turns, each working for eight hours. Two sets of the machinery—pumps, steam engines, and boilers—are provided, one being always kept in reserve, ready for use in case of any accident. Even in winter, when, from the lower temperature, the water contains the largest amount of oxygen, it is found that the stopping of the circulation of the water for only a few hours occasions manifest discomfort to some of the animals. The water is poured into the two centre tanks in an equally divided stream, and by a simple fall of a few inches from tank to tank it flows by two routes to the lowest tank, from which it passes into the reservoir below. This incessant circulation of the water constantly exposes fresh surfaces to the action of the air, by which oxygen is absorbed. But besides this, other small streams of water are made to forcibly enter the tanks from jets, by which a large quantity of air is carried down in very small bubbles. The removal of carbonic acid is accomplished by the vegetation which spontaneously makes its appearance in sea-water under suitable circumstances. It has been found quite unnecessary to introduce purposely any kind of sea-weeds, for the spores of low forms of vegetation are always present in the water, and they develop rapidly under the stimulus of light. Indeed, one of the difficulties of aquarium management is to avoid this excessive vegetation by limiting the light as much as possible, and yet leave sufficient illumination for the observation of the animals. The amount of light falling upon each tank is very carefully attended to at the Crystal Palace, and where it cannot be diminished sufficiently to check the overgrowth of vegetation, without at the same time interfering with a proper view of the animals, certain molluscs and fishes which live upon algæ are put into the tanks to consume them. This spontaneous vegetation is so vigorous that a comparatively small quantity suffices to remove from the water all the carbonic acid which it may derive from the animals and decomposing matters.

It should be mentioned that at this aquarium the water is never filtered, but its clearness is obtained merely by the perfect system of circulation. The unused food and excrementitious matters are oxygenated by the air which the water abundantly holds in solution—thanks to the surface exposed in its constant circulation, the injection of the jets of water carrying minute bubbles of air into the mass of water, and the gas given off by the vegetation. The whole process of purification is therefore chemical, and the success and excellent adaptation of the system may be judged from the fact that the water seen in masses 9 ft. deep appears perfectly clear and bright. The building is very cool in summer: even in extremely hot weather the temperature of the air within it is never higher than 68° F., and that of the water in the tanks never exceeds 63°. In winter the temperature of the air is maintained by hot-water pipes at from 60° to 65°, and the temperature of the water at about 55°. On winter evenings the aquarium is illuminated with gas, and the habits of many nocturnal animals can then be conveniently studied.

“All the animals in this aquarium,” says Mr. Lloyd, “have to be fed constantly; and as for the sea-anemones—of which there are in the aquarium over 5,000 individuals—every one of them has a morsel of food proportioned to its size, and according to the condition of the water, given it at frequent intervals with a pair of wooden forceps by an attendant who makes this his sole occupation—as these flower-like creatures, being so non-locomotive as to be almost absolutely fixed, cannot pursue their food, or in an aquarium obtain it in any other manner. They are here deprived of the action of the waves, which in the actual ocean brings them nutriment, which is arrested by their outspread and waving tentacles. The food consumed by a few of the animals now present in the aquarium is vegetable, consisting of green weeds (Ulva, Porphyra, Enteromorpha, &c.), but by far the greater number have animal food given them. This consists of shrimps, alive or dead, crabs, mussels, oysters, and fish, but they are never fed on butcher’s meat.”

The creatures known as “sea-anemones” are well represented in the Crystal Palace Aquarium. The observer cannot fail to be struck by their resemblance to flowers, from the radiated arrangement of their tentacles, and the beautiful colours they often exhibit. The opelet (Anthea cereus), Fig. 321, is perhaps the most beautiful among British species, and is a conspicuous denizen of the Aquarium, where its long green tentacles, tipped with lilac, are commonly seen expanded or twisting about like so many snakes. These tentacles are stretched out in search of food, and when by chance an unlucky shrimp or other suitable prey merely touches a tentacle, it is seized and held with remarkable pertinacity, the rest of the tentacles closing round it. The mouth of the creature, placed in the centre of the disc, then expands to an extraordinary size, and the prey is quickly lodged in the capacious digestive sac of the actinia, where the soft parts are soon dissolved, and the hard indigestible residue is ejected by the mouth. The tentacles of Anthea, and of other species belonging to the same subdivision of the animal kingdom, are furnished with an immense multitude of curious organs, which consist of cells or minute bags, containing coiled up within them a slender highly elastic filament. When these cells are compressed, the filament shoots out of its capsule to a surprising length; and it has been supposed that the adhesive power of the tentacles depends upon these filiferous capsules; while it is not improbable that some virulent fluid is also emitted from the cells, for the victims appear as if paralysed almost as soon as they are seized. Our knowledge of these animals has been largely extended by the opportunities of observing their habits which are afforded by marine aquaria.

To obtain the variety of animals requisite for stocking a public aquarium is by no means an easy matter; for the animals must be good specimens, in a healthy condition, uninjured by their capture or transport from the sea. The Crystal Palace Aquarium Company have at Plymouth a large pond, which communicates with the sea at every tide; and this, under the superintendence of the company’s resident agent, serves as a store for animals. Similar arrangements exist at Southend, Weymouth, Tenby, and other places. The specimens are brought to Sydenham by fast trains—special facilities being afforded by the railway companies for this purpose. The mode of carrying the animals depends upon their nature, and is sometimes a matter of no little difficulty. All fishes, except perhaps eels and blennies, must be carried in a sufficient bulk of water; and then the due oxygenation of the water and the removal of the carbonic acid can be but very imperfectly accomplished. A considerable mass of water is absolutely necessary in such cases, and the difficulties and cost of the transit are much increased by its weight. In warm weather the quantity of oxygen retained in the water is materially diminished, and under such circumstances the creatures would soon perish. On the other hand, in very cold weather the temperature may be so far reduced below that suited to their habits that death may also result from this cause. Crabs, lobsters, sea-anemones, sea-urchins, and similar animals can in general be carried without being immersed in a mass of water. These animals are placed in layers of wet sea-weed contained in baskets, so that the air has access to the moisture which covers the bodies of the animals, which is prevented from drying up by the humidity. As in this case the small quantity of water exposes a very large surface to the air, oxygen is plentifully supplied. Mr. Lloyd points out that it is owing to the readiness with which mere films of water are aërated, that it has been found possible to convey to Australia the eggs of salmon and trout, and hatch them there. They could not have been carried in water, but they were successfully conveyed when surrounded by a cool and very moist atmosphere. This mode of transmission is much more economical and convenient than the plan of carrying the creatures in water, and it is therefore resorted to whenever the organization of the animal permits.

Specimens of a very remarkable creature are, or lately were, exhibited at this aquarium in the lancelet (Amphioxus lanceolatus), Fig. 323, which animal itself is a comparatively recent discovery. It is about 2½ in. long, and although it is fish-like in form, it presents so many points of structure common to lower animals, that it is looked upon by naturalists as a link between the molluscs and the fishes—being the lowest of the latter in organization. The creature can hardly be said to possess a skeleton, the tissues representing that structure are so soft. It has no definite brain, but it possesses olfactory and optic organs of a rudimentary kind.

THE BRIGHTON AQUARIUM.

The Brighton Aquarium, already so well known as a place of popular resort, is a structure of considerable architectural pretensions, and is the largest establishment of the kind in existence. The idea of this undertaking appears to have originated with Mr. E. Birch, the engineer of the actual structure, who, having, in 1866, visited the aquarium at Boulogne, perceived that the construction at Brighton of a marine aquarium on a very extended scale offered every promise of commercial success. The promoters, in 1868, obtained from Parliament an act authorizing them to acquire a certain site for the aquarium, but imposing such limits as to the height of the structure that it was necessary to place the greater part of the building below the level of the ground, a matter involving considerable engineering difficulties. The aquarium is situated close to the Chain Pier and immediately below the cliff, the building being protected from the waves by a strong sea-wall of concrete and Portland stone. The building was definitely opened in August, 1873, while the meeting of the British Association was being held in the town. Its length is no less than 715 ft., and its average width 100 ft. The predominant element in the architectural style of the building is Italian. The following particulars as to the arrangement and dimensions of the various parts of the building are derived from the official guide-book:

Entering the gates at the western end, the visitor finds himself at the top of a flight of granite steps leading to the entrance court, 60 ft. by 40 ft. The front elevation of the building is 18 ft. high, and consists of five arches, with terra-cotta columns and enrichments. On the frieze running round the sides are the appropriate words, “And God said, Let the waters bring forth abundantly the moving creature that hath life.” On the northern side of the entrance court is the restaurant; and on the southern side a series of niches ornamented with vases. From this outer court, the entrance hall, which is 80 ft. by 45 ft., is approached through three doors. This is furnished with reading-tables and supplied regularly with periodicals, journals, and telegrams; while between the pillars supporting the roof are handsome pedestals, surmounted with large glass vases containing the smaller interesting marine and fresh-water animals, which would be lost to view in the larger tanks. In one of the recesses facing the entrance are four microscopes, in which specimens illustrative of subjects in natural history connected with the aquarium are constantly exhibited. To the north of the hall lie the general manager’s offices, the retiring-rooms, kitchen, &c.; and eastwards, in a direct line with the restaurant, is the entrance to the western corridor of the aquarium proper. This corridor, which contains a great many tanks, is the longest of any: it extends 220 ft., and is broken by a central vestibule, 55 ft. by 45 ft. The roof, which is groined, is constructed of variegated bricks, and rests upon columns of Bath stone, polished serpentine marble, and Aberdeen granite, the carved capitals of the columns having appropriate marine subjects. On each side are placed the first two series of tanks, twenty-one in number. These increase in dimensions from 11 ft. by 10 ft. upwards, the largest measuring over 100 ft. in length by 40 ft. in width, and holding 110,000 gallons of sea-water. This colossal tank is the largest in the building, and is devoted to the exhibition of porpoises, turtles, and other animals of large size. The next largest tank, 50 ft. by 30 ft., is immediately opposite.

The eastern end of the western corridor opens upon the conservatory, which serves as an approach to the rockwork, fernery, and picturesque cascade, and also to the eastern corridor. Some artificial rockwork, skirting the north side of the conservatory, is traversed by a stream of water, broken up at intervals so as to form numerous little bays and ponds, and utilized for the reception of seals and the larger reptilia. In the side-space between the conservatory and the second or eastern corridor are six octagonal table-tanks, of elegant design, for the exhibition of some of the smaller and more rare marine animals, and, at the eastern extremity, apparatus which serves to illustrate the hatching and development of trout and salmon. The entire length of this second corridor is about 160 ft., one side of the eastern portion, which is 90 ft. by 23 ft., being devoted to the exhibition of fresh-water animals. At the end of the corridor are situated the curator’s offices and the naturalists’ room, fitted with open tanks and all necessary appliances; and the engines, pumps, &c., for supplying the water, and keeping it constantly aërated.

The system adopted for aërating the water at the Brighton Aquarium is quite different from that used at the Crystal Palace. In the former the water is pumped directly from the sea into reservoirs formed under the floors, and capable of holding 500,000 gallons, which can be filled in ten hours. From these the water is pumped up into the tanks as required; but there is no general circulation through the system of tanks and reservoirs. Each tank is treated independently, and its water is aërated and kept moving by the injection of air at the lower part, effected by steam power.

The popularity of the Brighton Aquarium may be judged of from the fact that the average daily number of visitors is about 9,000, and that on some occasions nearly twice that number pass the turnstiles. Among the specialities of the establishment are herring and mackerel, which it has hitherto been considered impossible to preserve in confinement for any length of time. They are now thriving well in the Aquarium, although these fishes are both extremely impatient of confinement. The herring feed readily upon small shrimps, in catching which they display a wonderful activity. Fig. 324 shows the curious fish called the “sea-horse” (Hippocampus), from the singular resemblance of the front part of the body to a horse’s head, or, at least, to that form which conventionally represents the “knight” among a set of chessmen. The tail of the creature is prehensile, and enables it to cling to sea-weeds and other bodies. The sea-horses have thriven well in the Brighton Aquarium, and also in that at the Crystal Palace. The latest novelties are the Proteus from the dark caves of Adelsburg, axolotls from Mexico, the mud-fish (Lepidosiren annectans) from the Gambia, and the telescope-fish from Shanghai. Some of these creatures are of great interest from the circumstance of their forming the connecting-links between fishes and reptiles.

There are, therefore, now on view at the Brighton Aquarium specimens of three species of animals possessing a high interest for naturalists and others—not so much because their existence has been discovered in recent times, as because they are illustrations of the great law of gradation which exists in nature. Their position in the scale of organization is so intermediate between reptiles and fishes, that naturalists have not entirely agreed as to the kingdom to which these ought to be assigned. Fig. 326 represents Lepidosiren annectans, which has gills covered by flaps, and not exposed as they are in ordinary amphibious animals; and is provided with four fins, or rudimentary legs, according as the reader may choose to call them. The creature’s nostrils do not communicate with the mouth, but are merely two blind sacs, as in fishes. The Proteus anguinus, shown in Fig. 325, is an eel-like creature, only met with in the subterranean waters of the Grotto of the Maddalena at Adelsburg. It has four imperfectly developed legs, and gills reduced to mere fringe, while there are lungs extending nearly the whole length of the abdomen. The optical organs are entirely undeveloped, being represented merely by two specks. The axolotl, Fig. 327, inhabits certain Mexican lakes, and is remarkable for preserving, through the whole period of its life, the gills for aquatic respiration, which other amphibia possess in the tadpole stage only.

The mania for domestic aquaria which was at its height some years ago, and the great popularity of public marine aquaria wherever they are properly managed, express the real interest which is felt in the varied forms of animal life, of which the aquarium affords the opportunity of observing new and unknown phases. The progress of the science which treats of the organization of the animal kingdom has made rapid strides during the present century. Among the remarkable truths which have been acquired is the fact of the unity of the plan which pervades the animal kingdom. Each kind of animal has much in common with the kind above it, and with the kind below it: a certain community of organization pervades the whole, which is knit into one by the gradational forms which may be observed connecting, like links of a chain which cannot be broken, the more defined modifications from each other. It is their position in the scale of organization which, in the eyes of the philosophic naturalist, gives so much interest to some of the forms of life which have been figured above.

GOLD AND DIAMONDS.

It need hardly be said that gold and diamonds are named under nineteenth century discoveries in relation to the newly-found fields which have yielded these highly-prized substances in remarkable abundance.

GOLD.

This precious metal is met with in nearly all parts of the world, and its splendid colour, high lustre, the ease with which it may be wrought, and its property of ever remaining untarnished, have caused it to be greatly esteemed for ornamental purposes from the earliest historical ages. No doubt the store set upon gold is derived from its suitability for decorative uses; and its comparative scarcity enhances the regard in which it is held. Its use, as a standard of value, is justified by the general estimation in which it is held, and by the fact that the amount of labour required to obtain the metal is on the whole tolerably uniform. It is one of the few metals which are found in nature in the uncombined state, but its separation from the materials with which it is associated requires the performance of a certain amount of work, in whatever form the metal may occur. Its general distribution is another advantage attending its selection as the standard of value. It occurs in England and Wales; in Spain, in France, in Hungary, in Piedmont, and in other parts of Europe; in various localities in Asia; in both divisions of the New World; in the remaining quarter of the globe, where it was obtained even in very ancient times, for South-East Africa was probably the locality to which a naval expedition was despatched by King Solomon—“they came to Ophir, and fetched from thence gold.” Australia also has, in the last half of our century, yielded much gold.

Gold is never met with in regular veins, but in primitive or igneous rocks, or in deposits formed by the disintegration of these. In Australia the metal is associated with quartz, in slate rocks geologically equivalent to the Cambrian formations of England and Wales; and in California it is also chiefly found in material which has been formed by the wearing down of quartz and granite rocks. Before the discoveries in California and Australia most of the gold in circulation was obtained from auriferous iron pyrites. The first finding of gold in California occurred in September, 1847, when a Mr. Marshall, the proprietor of a saw-mill on the Sacramento River, observed some glistening grains among the sand in his mill-race. The news soon spread, and the inhabitants of the town of San Francisco, then numbering about two hundred persons, were greatly excited thereby. When it became known that gold was really to be found, multitudes flocked to California, the population of San Francisco rapidly increased, and at the present day the city contains nearly a thousand times as many inhabitants as it did at the time gold was first discovered. The annual value of the metal found in California averaged about £23,000,000 for ten years after 1851; but this subsequently declined to less than half in 1872.

Sir Roderick Murchison, the distinguished geologist, pointed out the great probability of the existence of gold in Australia many years before the precious metal was actually found. It has, however, been stated that gold was met with in Australia so long ago as 1788. Considering the mode in which the metal occurs, it seems strange that the emigrants who occupied the auriferous districts as agriculturists did not long ago discover the riches which Nature had scattered over the surface of the soil. No doubt, their attention was too much devoted to their sheep and cattle to notice the glittering particles which might be seen in the water-courses, and it would probably never enter their minds that the eagerly desired metal could lie exposed to view on the surface of the land. But the announcement of the discoveries in California induced men to look at the soil more attentively, and in April, 1851, Mr. Hargreaves appears to have found at Bathurst the first gold met with in Australia. Four months afterwards the metal was also picked up at Ballarat, Victoria, and the gold-fields so discovered proved even richer than those of Sydney.

The effect of this discovery on the colony of Victoria proved marvellous. The population, which in 1851 was 77,000, had in 1867 become 660,000; in the same period the land under cultivation expanded from 57,000 acres to 631,000, and the value of property rose enormously when the grazier’s estimate of its worth was replaced by that of the miner. The authorities of the colony from the first regulated the mining operations by enactments defining the rights of the miners to the “claims,” as the allotments of land for working upon are termed; and thus disorder and lawlessness were almost unknown. Fig. 329 will give the reader a notion of the appearance of a miners’ settlement in the Australian gold-fields in the earlier period.

The fundamental rocks in the colony of Victoria belong to the oldest series of strata. They answer to the Silurian formation which exists in Cumberland, Wales, and Scotland. Although the strata of the rocks are much bent, and they have been worn down by the action of water, they are as a whole but little altered, consisting chiefly of sandstones and shales. These strata are interpenetrated by innumerable veins of quartz, which vary in thickness from 1/16 in. to 150 ft. It is in these quartz veins that the gold is seen in its original matrix. The metal is sometimes in the form of grains or flakes, or in moss-like threads, embedded in the quartz; sometimes in the form of well-defined crystals, sometimes in rough lumps or nuggets. Fig. 330 shows three of the various modes in which the gold is found disseminated through quartz. Overlying the more ancient rocks with their auriferous quartz veins are various rocks of different ages; and as these have been in part formed by the wearing down of the older rocks, they also are in general auriferous, and contain the gold in detached pieces, varying in size from particles of fine dust to the huge nugget, containing 2,280 oz., or nearly £10,000 worth of pure gold, which was found at Dunolly.

The soil, which has been formed by the disintegration of masses of auriferous quartz, is full of gold, so that a patch of such soil 12 ft. square has been known to yield 30 oz. of gold by a very rough kind of washing to the depth of 1 ft. Soil of this kind has been carried down by rivers and streams ages ago; and the lighter particles having been carried off by the water, while the gold, from its greater specific gravity, remained at the bottom of the stream, the sands and gravel of these river-beds are very rich in gold. In many instances the ancient water-courses have been entirely covered by igneous rocks, such as basalt, which have flowed over the land in a molten state. The gold-miner often finds his reward in burrowing beneath these basalts and lavas, following the bed of the ancient river, and recovering its long-buried treasures.

The methods of carrying on the gold-seeking operations vary according to the nature of the deposit which is worked and the resources of the miner. The simplest, which was that most practised in the early days of the gold-fields, consists in throwing into a tub several shovelsful of the surface soil, and in pouring in water while the contents of the tub are stirred about with a spade. The lighter matters are washed away, but the gold by its great specific gravity remains behind. An improvement in this, but still a very rude process, is practised by aid of the cradle, Fig. 331, which is merely the trunk of a tree, hollowed out, and provided with transverse partitions and ribs. The auriferous earth is thrown into the upper compartment, which is then filled with water. The cradle is rocked, so that the water may wash away all but the gold and the heavy stones. Any particles of the former which may be carried out of the head of the cradle will be stopped by the ribs which cross the lower part. Machines for puddling by horse-power are now in use, and other contrivances have superseded the tub and the cradle in surface-washing. The auriferous earth obtained by excavating the soil from pits is washed in a similar manner, as is also the material reached by penetrating the deeper tertiary deposits, and by driving adits or tunnels along the ancient river-beds beneath the layers of basalt.

A mode of washing accumulations of auriferous earths by streams of water is employed where circumstances are favourable. A long inclined channel is constructed, and lined with boards; or, when the natural inclination of the soil requires it, a long trough is constructed and supported on trestles. The trough is made of sawn boards, 1½ in. thick, in sections 12 ft. long, and it has a width of about 1 ft., the sides being from 8 in. to 2 ft. high. The inclination of the troughs is from 8 in. to 24 in. in 12 ft., and depends upon the abundance of the water: the more water, the steeper is the slope. The bottoms of the troughs are crossed by a number of transverse bars, which arrest the auriferous particles in their descent. The sluice, or series of troughs, may be from 50 ft. to several hundreds or even thousands of feet in length, and the cost from £100 to £8,000. The earth is thrown in at the upper part of the trough, and it is gradually washed down, the water being allowed to flow in some cases by night as well as by day, but commonly in the day-time only, as the troughs must be watched, to see that they do not become choked up, and the soil washed out by the overflowing water. The run goes on for six or ten days, and then the current is stopped for a cleaning-up, which occupies from half a day to a day. For this operation the stream of water is stopped, and quicksilver is used to dissolve the grains of gold from the sand, &c., collected by the riffle-bars. The quicksilver is afterwards expelled from the amalgam by heat, and the gold remains as a porous mass.

Sometimes, instead of shovelling the earth into the troughs, it can be washed out of its position into suitable channels by means of a powerful jet of water. This mode of working, which is termed hydraulic jet sluicing, offers great advantages where the natural conditions admit of its adoption. In this plan, instead of bringing the auriferous earth to the water, the water is brought to the earth by a flexible pipe, like the hose of a fire-engine, from a reservoir about 200 ft. higher, and the stream is directed upon the material by a nozzle. This powerful jet of water is used to separate and carry away the earth to the head of a system of channels and troughs, like those already described. The hose has a diameter of 8 in., but the orifice of the nozzle from which the water issues is contracted, in order to increase the force of the jet. The hydraulic jet sluicing requires from three to six men to work it, and the material of a hill can be carried into the sluices in less time than a hundred persons could do it by spades. Immense quantities of earth are removed in this way, and fatal accidents are not infrequent from the falling masses burying the men who carry the pipe. The force of the jet of water itself is another source of danger, for broken limbs and even fatal injuries have often been caused by it. The number of accidental deaths occurring in hydraulic jet sluicing operations in the colony of Victoria is reported to average about 60 in a year. Material which has been worked before often yields a considerable amount of gold when the operations are repeated; and in localities favourable to the hydraulic jet system, the work can be carried on with little labour. In this way three men have been known to extract in one week from dirt washed for the third time, gold of the value of £330.

The gold which is embedded in quartz and other minerals, as shown in Fig. 330, is obtained by crushing the material in stamping machines, which are usually constructed with logs of wood shod with iron. In another form of crushing-mill two large cast-iron rollers are used instead of stampers. From the crushed material the particles of gold are extracted by amalgamation with mercury, which is afterwards removed by distillation.

The richness of the Victoria gold-fields may be inferred from the fact that, up to the year 1868, 36,835,692 oz. had been obtained, the value of which is no less than £147,342,767. The total value of the gold then annually obtained throughout the whole world is estimated at about 20 millions of pounds sterling. When gold was found so plentifully in California and Australia, it was supposed by some that its value as a monetary standard would be affected. This has not happened, although the prices obtained for the metal by its producers were considerably lower in the last decade of the century than about 1867. The total annual output of gold throughout the world is of course variable, and no doubt there are also variations in the demand; but, so far, the fluctuations have been relatively small, and there has been no such depreciation by excess of production as in the case of silver. Yet the increased production of gold after the discoveries made about the middle of the century was beyond precedent. It has been estimated that between 1850 and 1875 the total value amounted to £600,000,000, showing an annual average twelve times greater than that for the period between 1700 and 1850. Between 1875 and 1890 there was a falling off in the supply, the annual average becoming only £20,000,000 in value. But since the last-named date there has been a rise year by year, and at the close of the century the value of the gold produced throughout the world in one year may not be less than £40,000,000.

As already remarked, the distribution of gold is world-wide; and it has happened in recent times, that just as one source of supply has shown signs of failure, other fields have been discovered and have attracted thousands of eager seekers to new regions. So, when the Californian supply was falling off, there came the rush to Australia, where easily worked alluvial deposits or rich veins continued for years to reward the toil of the gold-finder, though in an ever-lessening degree, until in 1886 or 1887 the centre of attraction was shifted. But at a later period fresh discoveries in Australia again raised the productiveness of that quarter; and still more recently, the announcement of the existence of much auriferous deposit in the valley of the Yukon River (Klondyke), and in various localities of British Columbia, drew thousands to desolate and undeveloped districts, in spite of the extremities of hardship and destitution that might be endured.

The discovery of 1886 takes us to South Africa, a region with which also our next section is mainly concerned, and the scene of an activity unprecedented in the annals of gold-mining. From circumstances immediately connected with our present subject, the close of our century finds public attention intensely occupied with affairs at the austral extremity of the “dark continent.” The history of South Africa, from the time when, in 1486, the tempest-driven Portuguese mariner, Bartholomew Diaz, first struck its shores at the promontory he named the “Cape of Storms” (Cabo Tormentoso), and when, eleven years afterwards, the celebrated Vasco de Gama sailed round it on his memorable voyage to India, is one which, in many respects, presents features of peculiar interest. It is not our province to enter into details of these, but it may be stated that the “Cape of Good Hope”—the more auspicious designation which the King of Portugal substituted for that of Diaz—was, towards the end of the seventeenth century, colonized by Dutch and some French settlers, and afterwards Table Bay became a regular port of call for Dutch, English, and other ships trading to India. The Cape was taken possession of by the British in 1795, but restored to the Dutch in 1803, only to be three years after (1806) resumed by England, under whose rule “Cape Colony” has since remained. The abolition of slavery in all British dominions, enacted in 1833, was the occasion of great dissatisfaction to the descendants of the Dutch settlers, who inhabited isolated farmsteads, their possessions consisting chiefly of great herds of cattle, tended by slaves. These people, or at least the majority of them, resolved to quit the confines of British territory, and seek fresh fields and pastures new in unoccupied regions north of the Orange River, so that from 1835 to 1838 there was a continued “exodus of the emigrant farmers.” The story of the following years, with its exciting events and the vacillating policy of successive British Governments, must be perused elsewhere: suffice it to say here, that the settlements of the “emigrant farmers” had by 1854 established themselves into two separate States, nominally recognising Great Britain as the “paramount power,” but practically independent of it; for, at the last-named date, the autonomy of “The Orange River Free State” was acknowledged, and two years before that another section of the Boers, i.e. of the “emigrant farmers,” who had settled beyond the Vaal River, was absolved from British allegiance, and, restricted only by a claim of certain suzerain powers, was constituted into “The South African Republic,” of which the precise boundaries were at length determined by the “Convention of London” in 1884. This territory is usually called for shortness the Transvaal, and here in 1854 the existence of gold was first announced; but the Boer authorities at once prohibited further prospecting, fearing, and perhaps with reason, that the winning of the precious metal within their bounds might disturb their pastoral quietude. The Boer character has been the unique product of a race withdrawn for two centuries from contact with European and civilized culture, living in widely separated dwellings with scarcely other associations than cattle and enslaved blacks. The Boer is described as of a type which draws away from the enterprising man of modern times towards the primitive patriarch centred in his flocks and herds: he hates innovations, and greatly distrusts strangers; he would rather keep, in a box under his bed, any money he may possess than employ it, or his own energies, in developing the immense mineral resources of his territory, in which are included not only gold, but copper, silver, lead, iron, and abundance of coal.

After some years the prohibition against the exploitation of gold in the Transvaal was withdrawn, and several localities in the Republic subsequently became small capitals of gold-mining industry. The most notable were Leydenburg and Barberton, at which latter place as many as 10,000 gold-seekers were congregated when the discovery of 1886 drew most of them away. These communities were formed almost entirely by the influx of people from beyond the Boer boundaries, mainly, of course, English-speaking people from the Cape Colony, Australia, America, etc. Their operations were hampered by the Transvaal legislation, and impeded by the absence of adequate means of communication, which was a characteristic feature of the Boers’ unprogressiveness; nevertheless, gold-mining has been pursued in some of these localities ever since, though with varying fortune. What drew nearly all the gold-seekers of the Transvaal and of adjoining regions at once to the north side of the Vaal River was the discovery there of real gold mines. This was at a district within the Transvaal territory, named Witwatersrandt (= White-waters-ridge), the designation which has been reduced by abbreviation or affection to “The Randt”—or, anglice, the Rand.

It is singular that although the Randt district had been explored by expert prospectors between 1877 and 1891, the outcrops of the auriferous reefs entirely escaped their notice. But when the first hint of the existence of these deposits was bruited abroad, it was the Kimberley men who were foremost in surveying the spot. By the Kimberley men we mean those who so soon had by lucky chance lighted in 1870 upon the rich and apparently inexhaustible diamond mines, as related in our next section. By the time of the announcement of gold-finds on the north side of the Vaal River many of these men had become rich—very rich indeed. If they had been so disposed they might then have returned to their native countries with enviable fortunes, but it was just as the affairs of their diamond companies had been settled by consolidation on a satisfactory basis, and the spirit of discovery and adventure was still strong upon them, that they resolved personally to explore the new El Dorado. To a wild desolate region they proceeded, enduring there and on the track thither the like discomforts they had experienced in their earlier quest. But they took with them experts provided with all appliances for ascertaining the prospective value of the alleged discovery, and, when convinced of its reality, they purchased from the Boer possessors their land at the price demanded, and it was not long before they had chemists and engineers at work, having, at the cost of making their own roads, had brought to the spot the necessary machinery and appliances. The usual influx of workers, builders, speculators, etc., followed, and in a wonderfully short space of time a town sprang up where in 1886 there had been only a single poor farm. The town grew rapidly to the dimensions of a city inhabited by 150,000 people. Its name is Johannesburg.

In his book on South Africa, the late Lord Randolph Churchill, describing Johannesburg in 1891, says that it has much of the appearance of an English manufacturing town, but without noise, smoke, or dirt. “The streets are crowded with a busy, bustling, active, keen, intelligent-looking throng. There are gathered together human beings from every quarter of the globe, the English possessing an immense predominance. The buildings and general architecture of the town attain an excellent standard, style having been consulted and sought after, stone and bricks the materials, corrugated iron being confined to the roofs, solidity, permanence, and progress being the general characteristics.”

The Randt mines having drawn into the South African Republic great numbers of enterprising workers, who have acquired wealth and built cities, it would have been expected that they would have been permitted to acquire the ordinary rights of citizenship. The Boers’ character, however, has been manifested by their refusal of such rights, and by their exacting grievous imposts from these Uitlanders (Out-landers, or strangers), who, being for the most part of English race, are finding the injustice too hard to be borne, and greatly strained relations between the Transvaal and Great Britain have again supervened. Indeed, the situation has become so serious that it is feared actual war may result, and that is why, in almost the last year of our century, people are looking anxiously at the position of affairs in South Africa.

The geological conditions of the Randt are these: the upper series of beds in the Karoo formation, which extends over the greater part of South Africa, consist of quartzose strata, and in the district in question these are much broken, faulted, and variously inclined. They are interstratified with beds of sandstone and with the layers of gold-bearing conglomerate, of which last there are several parallel to one another and not far separated, ranging in their several thicknesses from 6 inches to 6 feet, the thickest being known as the main reef. These reefs form an oval basin,—that is, they dip with varying angle towards a centre, and crop out at their up-turned edges. Johannesburg is situated nearly 6,000 feet above the sea-level, on an elevated ridge, along which for 30 miles east, and nearly the same westwards, the northern outcrop extends, curving towards the south, while the southern edge of the basin appears in the Orange Free State, where it has been traced for a distance of 130 miles. There a shaft, sunk to the great depth of 2,400 feet, found the main reef with undiminished richness. The outcrop of the reefs stretches east and west for 130 miles, and the distance between north and south is 30 miles. From such data it has been inferred that the reefs contain altogether not less than 450 million pounds worth of gold. The conglomerate of these reefs consists of rounded quartz pebbles (which contain no gold), and pieces of sandstone and of argillaceous material, the whole cemented together into a very hard mass by iron pyrites. This last is the matrix in which the gold exists, in the form, for the most part, of minute scarcely visible crystals. To a depth of from 50 to 150 feet, air and moisture have acted on the pyritic matter, and the material of the reef becoming in consequence easily disintegrated, has yielded by mere mechanical treatment most of its gold, whereas by the same operations on the underlying hard, tough conglomerate, only about half its gold could be obtained. Hence, after breaking up the ore, the pyritic matter is sorted out and transported to the stamp battery, reduced to powder, from which about five-eighths of the contained gold is removed by quicksilver. The residue is concentrated by washing in a special machine called the “Frue vanner,” and the concentrates, after roasting in order to oxidize base metals, are subjected to the action of chlorine gas, by which the gold is converted into a soluble chloride, from the solution of which it is precipitated by ferrous sulphate. The tailings, slimes, and other residues are further acted on by a solution of potassium cyanide, which dissolves the minute remaining particles of gold, and from the solution the metal is obtained by electrolysis. By these supplementary chemical processes the total of the gold recovered from the ore is raised to 90 per cent. or more of all that chemical analysis shows to exist.

When it is said that the reefs are arranged in a basin-like form, it must be understood that this applies to their general disposition, for the regularity of geometrical shape does not belong to geological basins. There are considerable variations in the inclinations of the reefs: at some places they are nearly vertical, but generally they dip towards the centre at various angles, a slope between 25° and 45° being quite usual; and the inclination becomes less and less the deeper they go, so that it is presumed that the beds are level towards the centre of the basin. In the Randt the vertical shaft is rather the exception, the entrance to the mine usually following the inclination of the reefs, and the trucks of ore are drawn up sloping rails. From the inclined adits horizontal galleries are excavated right and left at various depths by which the main reef is worked, and there are cross cuts by which the reefs to the north and south may be reached. The most active district of the Randt is that which extends eastward of Johannesburg, where a long succession of tall chimneys and winding headgears together with the other appurtenances are visible. But there is nothing of a picturesque character about a gold mine, more than is presented by the aspect of an ordinary colliery.

The importance of the Randt gold-fields does not consist in the actual richness of the crude material, which indeed in places here and there cannot be profitably worked,—in mining parlance, it is not “pay ore.” It is rather the great ascertained extent of these gold-bearing beds and the general persistence of their character throughout that give to the Randt its unique character amongst metalliferous workings. This contrasts with the comparative uncertainty attending the exploitation of auriferous quartz veins, which occur in detached unconnected patches, that often end suddenly where least expected. There are in the Randt nearly one hundred companies working mines, and of these there are many that pay very handsome dividends on their original capital. A few pay 100 per cent., while a considerable number distribute 25 per cent. and upwards; so that some of these Gold Companies are amongst the richest and most influential financial houses in the world. The Randt is second only to the United States in the quantity it adds annually to the world’s production.

DIAMONDS.

In ancient times, and down to a comparatively late period, the only region from which were derived all the diamonds that found their way to Europe, was India, where Golconda was long celebrated for the productive mines in its neighbourhood, and for the high estimation in which fine specimens of their yield were held. In the seventeenth century these mines employed 60,000 persons, it is said; and in other districts of India diamond-seeking has also been carried on from time immemorial. A gradual decrease in the finds of Indian diamonds has long been observed, and the supremacy the East had so long enjoyed as the purveyor of gems was in the earlier part of the eighteenth century transferred to another hemisphere. In 1727 the diamond was first discovered in Brazil; or rather, we might say, was then first discerned there. For the gold-seekers in washing the sands of certain Brazilian rivers had found numberless specimens which they either threw aside as worthless, or, seeing them prettier pebbles than the rest, used them as counters in their card games; their true nature was not recognized, because the rough diamond has by no means the attractive appearance of the cut and polished brilliant flashing with refractive radiance. It must have been these last, and not diamonds in their natural state, that presented themselves to the imagination of the poet when he penned the line—

Or deep with diamonds in the flaming mine.

The announcement of some diamonds having been found in America had no effect on the prices in the Indian market, but the exports that soon after came from Brazil in great abundance quite changed the conditions of the trade, for in the first fifty years their value was estimated at no less than £12,000,000 sterling. As already stated, the presence of diamonds in Brazil was not recognized until 1727, and then by the accident of one Lobo, an inhabitant of the gold district of Minas Geräes, who had been in India and had seen rough diamonds there, observing the resemblance; he took some of the Brazilian stones to Lisbon, where their identity with the products of the Indian mines was established. But the European dealers, alarmed lest this discovery should depreciate the value of their stocks of Indian gems, spread a report that the so-called diamonds from Brazil were but the refuse of the Indian mines that had been sent to Brazil. This had the effect of stopping for a time the sale of the Brazilian diamonds; but the traders in these were not above taking a hint from their rivals—fas est et ab hoste doceri—for they carried their diamonds to Bengal, and there sold them as Indian stones at Indian prices. For nearly one hundred and forty years after this Brazil was by far the most productive diamond region in the whole world, and especially after 1754, when diamond-seekers congregated by thousands in the very rich fields of Bahia, a district of Brazil. Nor have the places above mentioned been by any means the only localities in Brazil where diamond-finders have been at work; but the production has decreased and has lost its relative importance by the South African discoveries that about 1870 caused an entire change in the diamond industry, and the high prices of the Brazilian gems no longer capable of being maintained, the fall in value has rendered the workings less remunerative than formerly. We may now pass over with mere mention, discoveries of diamondiferous districts in North America, Australia, and elsewhere.

While rejecting as entirely inapplicable and inexcusable by any stretch of poetic licence the epithet flaming for the diamond mine, we must question whether the word mine, that as the customary word we have continued to use, does not convey an equally false notion of the nature of the workings to which hitherto reference has been made. For these in most cases are nothing more than holes, very much like gravel pits in the side of a hill. The diamonds which have so far been in question are usually found among alluvial sands or gravels, the water-worn fragments of disintegrated rocks. These are in many cases carried down by rivers, and the diamonds under such circumstances are very frequently accompanied by gold; indeed, it is the search for gold that has in many cases led to their discovery. In the dry season of the year, which extends from April to October, the lessened currents of certain of the Brazilian streams are diverted from their course into canals, so as to leave dry the bed of the stream, and here the mud is dug out to the depth of six or eight feet or more, and transported near the washing huts, these operations being continued throughout the dry season. When this is over the digging is necessarily interrupted by great volumes of water that fill the rivers and streams, and the diamond-seekers devote their attention to washing the mud that has been collected. About one cwt. of this is placed in a long trough, and water is made to flow in, while the negro labourer stirs up the mass with his hands, until the water runs off clear, all the particles of mud having been washed away. The residual gravel is then very carefully examined, stone by stone, and any diamonds found are handed to the overseer, who watches all proceedings from an elevated seat. These Brazilian diamonds are mostly of a small size: occasionally, but very rarely, stones of quite exceptional value are found, but perhaps not one in 10,000. Formerly when in the Brazilian fields a negro slave found one of 18 carats, or more (18 carats = 72 grains), he not only obtained his freedom, but was rewarded with gifts, and for the finding of smaller stones commensurate rewards were given. The value of a diamond of the larger sizes depends upon so many adventitious circumstances that it would not be easy for any one to state the money’s worth of an 18–carat stone, but, considering too that the price increases in a more rapid ratio than the weight, we may to some extent draw an inference from the published values in 1867 of smaller Brazilian brilliants, perfectly white, pure, and flawless, when one of 5 carats (20 grains) in weight was priced at £350. As the rough diamond gives a brilliant of only half its weight, we may from the above assume an 18–carat stone to be worth in its finished state at least £1,000.

It may well be asked what are the qualities possessed by the diamond which have caused it to be so highly valued as an adornment all the world over; and here it will be proper to invite the reader’s attention to the chemical as well as to the physical character of the diamond. The most obvious and attractive quality of the cut brilliant is its unsurpassable lustre, which is due to its high refractive power. In a section of our article on light the subject of refraction has been dealt with, and an explanation given of the index of refraction. That of the diamond is the highest known, being 2·50 to 2·75; other precious stones have indices ranging from 1·58 to 1·78; those of glass and of quartz are between 1·50 and 1·57. It follows from the known laws of refraction that the limiting or critical angle is less for diamond than for other substances, as, for example, glass, for the posterior surface of a diamond will totally reflect all the light that falls upon it at any angle with the normal greater than 24°; glass will totally reflect only when the incidence is greater than about 42°: hence the diamond reflects from its farther surface about 64 per cent. of rays that glass similarly situated would allow to pass outwards without reflection.

Another property in which the diamond excels all other substances is hardness. It is the hardest substance in nature; for a diamond will scratch every other, but by none can it be scratched, except by another diamond. Not but that by the application of a file the edges of a diamond or brilliant may be notched and broken; but this would be through sheer mechanical force tearing the substance, and would be a test of brittleness, not of hardness. These two properties have not unfrequently been confounded, as when it was foolishly prescribed as a test for the genuineness of a diamond, that it should be placed on an anvil and struck with a hammer. No doubt many good and valuable stones have been sacrificed by this ignorant treatment. The hardness of the diamond does not prevent its being reducible to powder when so required. Again, diamonds are sometimes in such a condition of internal strain that very slight shocks are sufficient to cause them to separate into fragments. We read of diamonds that are suspected to be in this condition being packed for transmission within raw potatoes. The extreme hardness of the diamond secures it from all those accidental abrasions and injuries to which softer materials are liable, so that it does not deteriorate by age or use. It is unaffected also by any chemical substances.

In chemical composition the diamond is pure carbon, one of the most commonly diffused of the elementary bodies, as it enters into the constitution of the atmosphere, of all organic bodies, and of a vast number of mineral substances. Carbon in a less pure form also occurs naturally as graphite, plumbago or black lead, and in other conditions comes into ordinary use as already explained in our article on Iron. It was only towards the end of the eighteenth century that the composition of the diamond was demonstrated by the celebrated French chemist, Lavoisier, who actually burnt a diamond in oxygen gas, and found the resulting product to be carbonic acid gas, identical with that obtained by similarly burning a piece of charcoal. Soon afterwards another French chemist, Clouet, confirmed Lavoisier’s conclusion by producing steel from pure iron and diamond heated together, an experiment of much significance when considered in the light of the remarkable relation between these substances, which is one of the latest discoveries of our century. It should be observed that Clouet’s result implies a fusion of the diamond as well as of the iron in the act of entering into chemical combination.

Like nearly every solid substance of definite chemical composition, this pure carbon takes the crystalline form. The phenomena of crystallization are of the highest interest and beauty, for in them we see shapeless matter fashioning itself into definite and often perfect geometrical solids, as if it had been wrought by the hand of some mathematical artist. Every substance forms crystals of some one shape when the conditions are identical, and one essential condition for any crystallization is that the particles should be capable of free movement in arranging themselves, and this condition can occur only when the substance is in the state of liquid or of gas. Crystals are commonly deposited from solutions when the solvent evaporates or is cooled down; or they are formed when a fused substance solidifies. In either case the crystals are the larger and more perfect as they are allowed the greater time to form. Now, carbon in any of its conditions has been found to be absolutely infusible and insoluble, and therefore the origin of the diamond has long been a puzzle to scientific men, very diverse surmises having been propounded on this subject. Some have thought it was separated from carbonic acid by the action of heat, or of electricity; others, that the carbon had been gasified by subterranean heat; others, among whom were Newton and the German chemist, Liebig, believed that heat had nothing to do with it, but that the crystals slowly separated from vegetable matters (hydro-carbons) in the process of decomposition under some unknown conditions; others, that the diamond crystallized out from liquid carbonic acid, holding under pressure some unknown form of carbon in solution; others, that carbon was ejected by volcanic action in a fused state; and so on. We hope to show that the problem has at length been solved, and how.

The shapes of the natural crystals of the diamond must not be confounded with those of the cut brilliants. The most frequently met with of the former is the octahedron, or eight-sided figure, such as would result from two square pyramids joined base to base, the triangles forming the sides of the pyramids being of such a height that the three pairs of opposite points are equidistant one from another, so that the octahedron enclosed in a cube would have an apex in the middle of each surface of the cube. There are other shapes of diamond crystals, but they are all related to the cube, that is, they are all obtainable from the cube by successively slicing off edges and angles. The natural diamond sometimes has as many as 48 faces formed by such a process. This will easily be understood by the reader if he will take a cube of common soap and perform on it these operations gradually with a sharp knife, taking care always to make the new faces he produces equally inclined to the adjoining ones. He may begin by cutting off a tiny piece from one corner of the cube, forming a small equilateral triangle; then let him do the same at two opposite corners, and again at all the eight corners. Then he should make the cuts larger and larger, always producing equal sized equilateral triangles so long as these can be formed. In every case he will have shaped out such forms as belong to diamond crystals. Instead of this, he may pare off one or more edges of the cube, or he may in various ways combine the two operations, and he will probably be surprised at the variety of forms producible in this manner, all derived from the original cube and all representing possible forms of natural diamonds, and indeed those of any substance that crystallizes in the cubical system. A model of the diamond octahedron can be readily made from the description already given, and the whole series of operations will constitute an elementary but very instructive lesson in the science of crystallography.

Diamonds are liable to occur with every imaginable distortion, so as to be scarcely recognizable by their external form. A very pure smooth uncut diamond, belonging to the Rajah of Mattam in Borneo, is shaped exactly like a pear, two inches in length. By the way, battles have been fought for the possession of this gem, and it is said that,£200,000 was vainly offered for it. The diamond, notwithstanding its hardness, splits with comparative ease in certain planes, and by such cleavage (a property common to all crystals) the octahedral form commonly emerges. It was not until the middle of the fifteenth century that the art of cutting the diamond into regular facets was practised, and this can be done only by the aid of diamond powder, prepared by crushing fragments and faulty stones in a hard steel mortar. The first operation is to split the stone by its natural cleavage, and the rough facets so produced of two diamonds are ground together until they are quite smooth. The grinding of other facets and the polishing are effected on horizontal discs of steel making 2,000 revolutions per minute, and overspread with diamond powder mixed with olive oil.

The external surface of the diamond in its natural state is often very rough, the stone being always coated with a more or less opaque crust, so that its translucent interior is concealed or veiled; but when the reflection from its inner surfaces pierces this veil it glows as if lighted from within, giving that peculiar appearance which is called its “fire.” The surfaces of the diamond crystals are very often curved instead of being flat, and the dodecahedral shape, when this is the case, takes on an almost globular appearance. Diamonds of all colours are found, as well as the highly esteemed colourless stones. Yellow ones of various tints are frequent,—orange, brown, and pink are not very rare; but red, green, blue, and black are almost unique, at least in a condition to form large and perfect gems, and are accordingly much prized. The black diamonds found in Borneo are so hard that ordinary diamond powder has no effect whatever upon them; they have to be manipulated with their own dust. The nature of the substances that impart these colours to the diamond has never been made out; they must be excessively small in quantity. When a diamond is burnt in air or in oxygen gas by aid of a large burning glass or otherwise, an extremely minute quantity of ash remains, and this often retains the shape of the stone, in the form of a most delicate network; and of the composition of the ash, this much has been made out: it contains silica and iron. We shall find that the presence of the last named element, although but in the merest trace, is not without significance.

The purely utilitarian uses of the diamond are few, but of importance. The most familiar is in the glazier’s tool for cutting glass, and in connection with this we may mention a fact not generally known, namely, that though any point of a diamond will scratch glass, it is only by a natural point of the crystal, and that point of a certain shape, that glass can be cut. Another kind of diamond, valueless as a gem, has been turned to good account in Major Beaumont’s invention, described in our section on Rock Drilling Machines, to which the reader is referred. Minute diamonds are employed for writing on glass, for very fine engraving, etc.

Having now said sufficient about diamonds in general to give the reader an interest in the subject, and yet but little more than was needed to impart the information necessary for following the further development of the theme, we approach the discoveries in this connection which have specially distinguished our century. We must transfer the reader’s attention to South Africa, and if he can refer to any recent map of that region, particularly to one showing its physical features, it will be of advantage.

In 1867 some children, playing near the banks of the Orange River, found what they thought to be merely a pebble prettier than the rest. A neighbour seeing the stone in the children’s possession, obtained it from their mother for a trifle. It passed through several hands, and was bought at last by the Governor of the colony for £500. The discovery shortly afterwards of other diamonds in the same locality attracted numbers of persons to the district, and especially to the banks of the Vaal River, which speedily became the scene of a great search for diamonds. Though this search was confined to merely the surface of the soil, it was attended with considerable success, and many fine diamonds rewarded the diligence of the eager seekers. One of the most remarkable stones for its great size, which equalled that of a walnut, was discovered by a Kaffir. When this gem had finally reached the hands of Messrs. Hunt and Roskell, of London, its value was estimated at no less than £25,000. News of these discoveries having spread, a rush set in for the diamond-fields of the Vaal River, and the banks of this stream soon presented an animated spectacle. Europeans flocked to the spot, London jewellers sent agents, and the inevitable Jews appeared on the scene to purchase the precious gems from the lucky finders. It turned out that many of the larger stones had a slightly yellow tinge, varying in different specimens from the palest straw to a decided amber colour, and, as this detracted greatly from their value, no little disappointment and loss were sometimes experienced when the gems came to be sold in London and Paris.

One of the first settlements which sprang up on the banks of the Vaal River was a place called Pniel, of which the reader may form some idea from Fig. 332, which is copied from a sketch actually taken from the windows of Jardine’s Hotel. It was then only a little straggling village, chiefly of wooden sheds or corrugated iron erections, with but two or three more substantial structures. The diamonds which were found in this neighbourhood were obtained from gravel which lay on the slopes of the hills rising from the river. The mode of conducting the search for diamonds in these gravels was simple enough. The first operation was the washing of the material, in order to remove sand and dirt, and this process was usually performed at the margin of the river, where the gravel was brought down in carts and deposited in a suitable place, at which a cradle was erected. The cradle was simply a strong wooden framing sustaining sieves of wirework or perforated metal, placed one above the other, those at the top having the largest meshes, so that the lowest would only permit sand or very small pebbles to pass through. The cradle was capable of receiving a rocking movement, and while the gravel was thus sorted, water was freely poured on the uppermost layer, so that the stuff was in a short time thoroughly cleansed and sorted. When this had been accomplished, the gravel was thrown in successive lots on a table, at which the digger sat and rapidly examined it for diamonds by help of a flat piece of wood or iron (see Fig. 328). The larger gems were readily detected, and indeed could be picked out from among the pebbles on the sieve before the stuff was thrown on the sorting-table. Crystals of quartz, which sometimes glisten among the mass, often excited groundless delight in the bosom of the inexperienced worker.

On the payment of certain fees, the digger obtained a “claim,”—that is, he acquired the right of working an assigned portion of the soil. But if the claim had been left unworked for a week, it might be, in mining parlance, “jumped”—that is, any person might take possession of it, or jump into it, on procuring a proper licence.

Since the first rush of diamond-seekers to the river-banks, the stones were abundantly found elsewhere, namely, at the “dry diggings,” where the soil, dug out with a pick or shovel, was sifted first through rough sieves, afterwards through sieves having fine wire meshes The sieve, in such cases, was often suspended by thongs of hide between two upright poles, in the manner represented in Fig. 333. The miner was thus enabled to swing the sieve rapidly about, until the sand and dirt were separated, when the remaining gravel was emptied on the sorting-table in the manner before described. As the idea was formerly entertained that diamonds lie only on or near the surface of the soil, the early miners seldom penetrated more than a foot or two beneath the surface. But it was discovered that, so far from it being true that diamonds are present in superficial deposits only, the finest stones are met with at considerable depths to which no defined limit can be assigned; thus in sinking a well large diamonds have been found at 100 ft. below the surface. When these facts became known, many of the abandoned claims were worked over again down to a depth of 30 ft. or 40 ft.

The rapid rise of localities under such conditions may be illustrated by the case of Du Toit’s Pan, which is the centre of a dry-digging district, and grew in a wonderfully short space of time from nothing to be a town hiving several large hotels, two churches, several public billiard-rooms, a hospital, and a theatre. In 1871 the claims at this place, each 30 ft. square, sold at prices varying from £1 to £50–-the person who worked a claim paying also a small monthly sum for the licence. But those who were lucky enough to have obtained the first possession of the claims at another famous dry-digging locality, named Colesberg Kopje, at the cost of only the licence at 10_s._ per month, must have been still more fortunate, and have realized an enormous percentage on their investments; for, four months afterwards the ruling prices at the last-mentioned place were £2,000 and £4,000 per claim. This great increase in value cannot be wondered at, if the accounts related of the value of the diamonds found here are true. For instance, it is stated that one individual, who just before the great rush had bought a claim for £50, found in it diamonds worth £20,000. Colesberg has become a populous town, with good buildings and regularly laid-out streets, while a great camp of tents and other temporary structures still surround it on all sides.

At all the towns above-mentioned newspapers were published, relating chiefly to matters interesting to the miners—giving, for example, lists of “finds,” with the names of the lucky finders. It is curious that the term “diamondiferous” has, in these localities, come to be used as a general term denoting excellence of any kind. Thus, when it is desired to apply an epithet of superlative praise to a pickaxe or to a piece of furniture, this significant adjective is made use of; and a salesman in the diamond-fields will not hesitate to speak of diamondiferous coats and trousers!

It will be seen that the early diamond-seekers at the Cape followed very primitive methods, by simply washing in sieves the gravel and sand shovelled out of the river banks; and indeed, it was only when, about 1871, they began to dig deeper that their working seems entitled to be called mining. The “dry-digging” operations began at the since famous Du Toit’s Pan, by the circumstance of a Boer farmer finding to his great surprise diamonds sticking in the walls of his house, which had been built of mud. When the locality of this mud was examined by digging, more diamonds were found; and when the excavation was continued downwards, still more. At this place and at four others, all within a circuit of less than four miles diameter, have been developed the richest diamond mines in the world, throwing into the shade the produce of all the river gravel washings; and what is still more remarkable, showing no signs of exhaustion after nearly thirty years of working, but rather the contrary. The locality soon presented a scene of the most active industry, and it was not long before the town sprang up which has since become celebrated all the world over—Kimberley, the diamond capital. Kimberley is situated at the northern part of the British territory known as Cape Colony, not far within its boundary, and about 14 miles from the Vaal River, in Lat. 28° 43´ S., Long. 24° 46´ E. It lies in a north-easterly direction from Cape Town, at a distance of about 550 miles. When the existence of diamonds at the Cape became known, a great influx of strangers seeking fortune set in to a land that had failed to offer the attractions to colonists that America and Australia did. Before the establishment of the overland route opened a more direct way to India, China, and Australia, Cape Colony owed whatever importance it had to its position as a provisioning and coaling station for ships and steamers. As a British settlement it was little regarded, and its somewhat somnolent condition would have been deepened by the opening of the Suez Canal in 1869, had not the diamond discovery in that very same year brought about a great change. But the early diamond-seekers found their land of promise a wilderness without roads and without habitations, for the development of civilization did not then extend far from the coast. It is true, that here and there, at great distances apart, a few primitive missionary stations might be found, like that of Pniel shown in one of our cuts, which also represent the inhospitable aspect of the country. One cannot but admire the pluck of the adventurers, who, though unversed in their quest, encountered in its prosecution prolonged toils and many hardships. But they were young men, and their perseverance gained its reward. They came from all parts: from Britain, from America, from Australia, from Germany, even from Russia.

The finding at Du Toit’s Pan, and at contiguous places, of diamonds at some depth below the surface of the soil, led to geological examinations of the district, which ultimately resulted in discoveries of the highest interest and importance, as will now be explained, with first a few words about the external features of the country.

A traveller directing his steps northward from the sea-shore at almost any part of the southern coast of Cape Colony will be faced by several successive ranges of mountains, or what will appear to be such, running more or less parallel to the coast, and of no great elevation. When he has reached the summits of these heights he will not find corresponding declivities on the northern side, but nearly level plains, bounded northwards by other similar ranges. Supposing him to set out at a point, say, 150 miles east of Cape Agulhas (the most southern point of Africa), he will, about 50 miles from the shore, have reached the top of the third of the great escarpments which rise up like the stages of a gigantic terrace, and having thus gained the ridge of the Black Mountains, he will see one of these almost level plains stretching before him a breadth of 80 miles, for the most part arid and inhospitable, with a much greater length east and west, and bounded on the north by a portion of the range of elevations that in an almost unbroken line runs through Cape Colony to Delagoa Bay nearly parallel with the coast, at a distance from it between 100 and 150 miles. This extensive plain is known as the Groot Karoo (Great Karoo),—karoo being the generic name for such plains in South Africa. After crossing the Great Karoo, our traveller, on mounting the last far-reaching step of the Brobdignagian staircase, may find himself on the summit of the Nieuveldt Mountains, at an altitude of nearly 10,000 feet above the sea-level, attained in several widely separated stages within a distance of 140 miles. From the summit of these elevations there is no descent by terraces northwards, but the high tableland or plateau stretches away for hundreds of miles, descending by only a gentle slope towards the Orange River, but maintaining an average altitude of nearly 6,000 feet, and extending far beyond the Orange River towards the Equator. Kimberley is situated about 50 miles north of the Orange River, and 4,042 feet above the sea-level.

It was soon observed that the Karoos had common geological characters, consisting in a certain series of shales, coal, limestones, etc., and this series naturally came to be called the “Karoo formation,” just as we in England speak of the Wealden formation, etc.; and it was found that it extended over a greater part of Central South Africa, covering an area of at least 200,000 square miles, with an estimated thickness of 5,000 feet. The reader need not imagine that a boring nearly a mile in depth had to be made for the ascertainment of this last dimension, if he will remember what has been said in the last paragraph about escarpments of the rocks looking everywhere towards the coast. There is reason to believe that these beds were originally the sedimentary deposits of a vast fresh-water lake, or inland sea, far back in geological times. But here we need only concern ourselves with the development of the Karoo beds about Kimberley. There the ground is covered by a sandy soil of a red colour, for it contains much iron. Below this there is a layer of decomposed basalt, also containing much iron, its thickness varying from 20 to 90 feet. This lies upon a bed of very combustible shale, with carbon and iron pyrites, 250 feet thick, which from its great development here is known as Kimberley shale; then, after a conglomerate stratum 10 feet thick, is found a very hard compact rock, resembling hornblende, extending 400 feet downwards, and resting on another hard rock of quartz, also 400 feet in depth. These beds are nearly horizontal, but dipping a little towards the north. In speaking of them collectively we may use the local term of the miners and call them “the reef.”

Now, there are a few certain spots near Kimberley, and two or three elsewhere, in which the strata forming “the reef” are not found, but something quite different. These may be compared to large dry wells, extending vertically downwards to unknown depths, which have been filled up with matters from below. They are called pipes, but they are uncommonly large ones; for though of a somewhat irregular circular or oval shape, their diameters range from 200 to 500 feet. Nor must it be supposed that the enclosing reef presents itself as a smooth wall, as the name “pipe” might suggest. These pipes are true diamond mines. They are believed to have been formed by an eruptive action originating from below at a great depth, and this was not by the escape of red-hot lava or other molten rocks, but by that of steam or other gases. It is known that the eruptive forces acted from below, for the edges of some of the strata are seen in places in the walls of the reef that surround the pipes to be turned a little upwards. It is known that the erupted matter was not molten lava or rock, for the shale and other strata show none of those changes of character near the pipes which would have resulted from igneous action, and for the same reason the gas or steam that escaped by these pipes could not have been highly heated. It must therefore have forced its way through the strata by enormous tension or pressure, and this either at one terrific outburst or possibly by the gradual enlargement of smaller volcanic chimneys. These blow-holes are filled with a mixture of subterranean débris, as if mud had been forced up from below, carrying with it an extraordinary variety of rock fragments and crystallized minerals. These are embedded in a mass of a bluish-green colour much resembling indurated clay (but nearly as hard as ordinary sandstone), and this on long exposure to the weather crumbles down to a yellow friable substance. More than eighty different kinds of minerals of the volcanic class have been found in this breccia, as it is termed by geologists, and it is remarked that these fragments could not have been exposed to any great heat, for their edges show no signs of fusion. There are also embedded in the agglutinating substance large masses of the surrounding strata, sometimes having an area of several thousand square feet, and these are called in miners’ parlance “floating reef.” The cementing material is named “blue ground,” and the same when crumbled down by exposure is known as “yellow ground.” These colours are due to oxides of iron, which in the unaltered ground give the blue-green tint, being lower oxides; but are converted by absorption of oxygen into yellow and higher oxides. The upper part of the pipes is filled to a depth of about 70 feet with “yellow ground,” produced by the penetration of atmospheric influences. Blue ground and yellow ground alike contain diamonds, and the yield of these is pretty regular at all depths in the same mine (some have been explored down to nearly 2,000 feet), although it varies considerably from one mine to another, and in some the east side is often richer than the west. Thus in one load (1,600 lbs.) of ground from Du Toit’s Pan, in 1890, the quantity of diamonds found averaged less than 2 grains (0·5 carat), while Kimberley yielded 1·25 to 1·5 carats (5 to 6 grains). It is singular that the stones from mines quite close together are so distinctly different in character, that the Kimberley merchants can tell at once the source of any particular parcel. This would indicate that the blue mud was not forced up the several pipes at one and the same time, carrying with it diamonds from one birthplace.

The existence of the diamondiferous pipes is pointed out by no indication on the surface, which is covered nearly uniformly with the red sandy soil already spoken of; although indeed the site of the Kimberley mine was marked by a slight elevation, and that of Du Toit’s Pan by one of the depressions there called pans, which, at least in the wet season, are receptacles for surface water. The Wesselton mine, which was found only in the last decade of the century, about a mile from Du Toit’s Pan, also showed a surface depression, and that had been utilised as a depositing place for dry rubbish. At a later period the “Leicester mine” was accidentally discovered 40 miles away. At Jagersfontein, in the Orange River Free State, 60 miles from the Kimberley mines, is another pipe which yields the finest diamonds of any, commanding prices nearly the double of those paid for the De Beers and Kimberley gems, being in fact their nearest commercial rival. The proprietorship of the Kimberley group having in 1889 become united in the hands of one company, known as the “De Beers Consolidated Mines,” this company is able practically to control the diamond market, as it has sometimes turned out in a year as much as 3 million carats of diamonds, which sell for about £3,500,000. Up to the end of 1892, 10 tons of diamonds had been derived from these mines, representing a value of £60,000,000 sterling. In 1895, the De Beers Company sold diamonds to the amount of £3,105,958, the total expense of working for that year being £1,704,813,—the net profit was £1,401,145. The effect of consolidating all the Kimberley diamond interest into the De Beers Company has been to give an almost complete monopoly to this last, which has however found it advantageous to restrict its production to an annual output of about £3,000,000 in value, as the putting of a larger quantity of diamonds on the market would cause lowering of their price, and a diminution of the profits all round. The reason is, that though the world at large annually spends between 4 and 4½ million pounds sterling in the purchase of diamonds, yet it would not by a reduction in their price be induced to spend proportionately more. The company are sufficiently supplied by only two of their mines, the Kimberley and the De Beers, the expenses of working these being also relatively smaller than is the case with the others. It may be of interest to compare the quantities of diamonds that have so far been produced from the world’s greatest fields, leaving out Borneo, the Ural Mountains, Australia, etc., as comparatively insignificant. Estimated produce of India, from the remotest period, 10 million carats; of Brazil (since 1728), 12 million carats; of South Africa, in only 19 years, 57 million carats.

At the time of the discovery of the Kimberley mine (July 1871) it was divided into about 500 claims, each 31 feet square, and between these were roadways across; but when the claims were excavated to a depth of 100 feet or more the roadways became unsafe, and, the “blue ground” underneath them being too tempting always to be left for their support, they began to fall in, and the mine was often threatened with ruin from this cause. The state of things became still worse when the unsupported walls of the “pipe” itself began to collapse, so that by 1878 a quarter of the claims were buried in the ruins of the reef. These falls continued, and although very large sums were year after year expended in removing the fallen reef, the cost amounting in 1882 to 2 million pounds sterling, it was found at last that very few of the claims could be regularly worked, and when in 1883 a tremendous fall of 250,000 cubic yards of reef took place, covering half the area of the mine, it became necessary to adopt another mode of working, namely, a regular system of underground mining. Vertical shafts were sunk at a considerable distance from the pipe itself, and tunnels from these carried through at different levels, with a system of galleries so arranged that all the “blue ground” is removable without danger to the miners. The whole mine is illuminated by electric lights, and the different kinds of labour are carried on by distinct sets of workmen, some of whom drill holes for the reception of dynamite cartridges, others shovel the material into trucks, others again wheel the trucks along tram lines, which converge to a space where their contents are discharged into skips holding four truck loads, in which they are hoisted to the surface at about the rate of 400 loads per hour. This goes on day and night, the miners working in three shifts of eight hours each. About 8,000 persons are employed, 6,500 of whom are blacks.

Fig. 334_a_ is a sketch section of the Kimberley diamond mine, approximately to scale, and a glance at this will elucidate the foregoing description. The thick vertical and horizontal lines show the positions of the shafts and galleries that have at various times been excavated, the lowest gallery being connected with a shaft a considerable distance from the pipe, towards the right, but out of the range of the sketch. The fringed lines at the top, with dates, give some idea of the forms of the excavations until the final fall of reef that determined the resort to subterranean working.

When the “blue ground” has come to the surface, how are the diamonds to be extracted from the hard mass? how can a stone of a few grains weight be found amongst 1,600 lbs. of miscellaneous matter—a thing perhaps not larger than a peppercorn in four cubic feet of compact material? The “blue ground” is spread out on levelled and carefully prepared areas called “depositing floors,” and there, after a few months’ exposure, all but the very hardest pieces crumble down, the atmospheric action being accelerated by turning the material over with harrows, and by occasional waterings. The “blue ground” from the De Beers mine requires at least six months of this treatment, and it contains a certain proportion of refractory lumps that would not disintegrate in perhaps less than two years. These lumps are coarsely crushed between rollers, and the fragments are spread over slowly moving tables, from which any larger diamonds are picked off; the fragments left go through smaller crushers, and are subjected to still greater concentration. The depositing floors of the De Beers mine are laid out as rectangles, 600 yards long by 200 yards wide, each holding about 50,000 loads. They occupy several square miles, and as the “blue ground” spread upon them is always one of the most valuable assets of the company, the quantity of it forms an important item in the balance-sheets, and the amount that can be realized from it can be estimated with sufficient closeness, on account of the nearly uniform distribution of the diamonds. Thus in June 1895, the 3,360,256 loads then on the floors were put down as equivalent to nearly 1 million pounds sterling. When the “ground,” thoroughly weathered, has become yellow and friable, it is transferred to the washing machinery, by which about 99 per cent, of the original non-diamondiferous material is removed, and, thus concentrated, the gravel is together with the mechanically crushed material submitted to the action of a machine called the pulsator, where the gravel is first assorted into sizes by being turned about within an inclined iron cylinder perforated with several stages of round holes of diameters successively of 2, 3, 4 and 6 sixteenths of an inch. The pieces that are too coarse to pass through the largest holes are taken to the sorting house direct; but the stones that have passed through the cylinder drop according to their sizes into four separate sieves called at Kimberley jigs, from the well-known mining term jigger, applied to a man who washes ores in a sieve. The several jigs into which passes the now assorted gravel have screens with meshes corresponding to the holes in the cylinder; and by a very ingenious arrangement the concentration is carried to the point at which the diamonds can be individually picked out. The “jigs” themselves do not move, but all over the meshes of the screen is spread a layer of leaden bullets, which prevent a too rapid passage through the screens, while the material is kept moving in water, by that liquid pulsating or emerging in quickly succeeding gushes from below the meshes, and thus carrying off the lighter matters, while those of greater specific gravity, including the diamonds, work their way downwards between the bullets and through the meshes, and are received in boxes which are periodically carried to the sorting house.

When the now much concentrated diamondiferous gravel reaches the sorting house, the remaining operation consists merely in picking the diamonds out. But simple as this operation is, it has to be conducted systematically. In the sorting house are long tables covered with plates of iron, and placed in a good light. Upon these is thrown the wet gravel, but not promiscuously; the different sizes being set apart, the sorter spreads out the heap before him with a flat piece of zinc, picks out the diamonds and drops them into a small box. Only white men in whom confidence can be placed are allowed to deal with largest sized material, for this offers the strongest temptation to purloiners, as in this of course the most valuable stones are met with. This material, after the first search, is submitted to the scrutiny of another person, to see that no diamond has been overlooked; but the smaller assortments are examined by blacks, who are closely supervised by white men. The value of the diamonds occasionally sorted out in a single day may reach £10,000.

At the diamond mines little trust is reposed in the honesty of the blacks. Below ground and above ground they work under the constant surveillance of white men, and they live in “compounds” which are spacious areas—perhaps of 20 acres in extent—enclosed by lofty iron fences, and containing long rows of corrugated iron erections divided into rooms, each appropriated to a score of natives. Food, etc., is supplied from a store at less than ordinary prices, and the company find fuel and water gratis, and provide a well equipped hospital and medical attendance. There are swimming baths, and ample recreation grounds for dancing, etc. The natives of each of the many tribes keep by themselves apart, and follow their own fancies. They receive good wages, and some of them save money. They are not allowed out of the “compound” or the mine, except to work on the depositing floor, which they do under guard. They accept their restrictions voluntarily, making agreements for a certain term, three months being the least. Those who leave, as many do to spend their earnings, often “not wisely but too well,” usually return. The depositing floors are surrounded by fences 7 feet high, unscalably and impenetrably armed with barbed wire; and as here robbery would have the readiest chance, where the largest stones might be met with, extraordinary precautions are taken, watch and ward being maintained by day and by night. Not more vigilantly did Cerberus keep the entrance of Pluto’s domain, nor the wakeful dragon guard the golden apples of the Hesperides, than the patrols observe the depositing floors. At night powerful electric searchlights are made to play across the enclosures, so that unauthorized movements can scarcely escape detection. Besides these provisions against theft, the laws of the Colony prohibit any attempt at illicit dealing in diamonds, under a penalty of two years’ penal servitude.

The maximum penalties for contravention of the Diamond Laws are, however, much more severe, and that to an extraordinary degree. Thus any unlicensed dealer is liable to a fine of £1,000, or fifteen years’ imprisonment, or both. And the authorized dealers are required to keep a most minute record of all their transactions, to send a copy of it every month to the head of the police, and to produce it when required. It is needless to say that extraordinary precautions are taken to prevent the native workmen from secreting diamonds. And any person even finding a diamond, and neglecting to report the circumstance to the proper quarter at once, is liable to the pains and penalties above mentioned.

The “blue ground” was at first supposed to be the original home of the diamond, within which it had somehow taken its shape. But no satisfactory explanation was forthcoming as to the state of the carbon before its solidification into the crystalline form. The more general opinion has been in favour of a volcanic origin due to very high temperature; and although the “blue ground” itself is clearly not the ordinary erupted matter of volcanoes due to igneous fusion, the geology points to the district having been the scene of very active and extensive volcanic energies at more than one remote period, for the bed of the Karoo inland sea has been several times covered by level sheets of molten matter extruded somewhere from below; but not through the “pipes,” which were blown out ages afterwards. The strata of basalt and of hornblendic mineral, which extend horizontally over great areas in the Karoo formation, are of igneous origin, as are also some nearly vertical dykes of trap rock, about 7 feet wide, that are found traversing the “blue ground” in certain directions. These intrusive dykes are of course more recent than the formation of the blue ground, and that is itself later than the production of the pipes. The fact of many fragments of crystals being found in the “blue ground” does not comport with the theory that supposes it to be the matrix; and besides this, many of the diamonds show scratches, and as these are producible only by other diamonds, it would appear that they must all have travelled in company, some part of their journey at least.

Carbon in any form is quite infusible at the highest temperature we have hitherto been able to produce, although an incipient softening under the influence of the electric arc has been suspected. Professor Dewar, an English chemist, basing his data on analogies with other substances, and on purely theoretical grounds, has calculated that the melting temperature of carbon is near 3,600° C. (6,512° F.), and that it cannot remain in a liquid state at a temperature exceeding 5,527° C., when its vapour would have a tension equivalent to a pressure of 15 tons on the square inch. So far as these deductions are correct, both the melting point of carbon and the boiling point of its liquid must lie within the range of temperature expressed by 3,600° C. and 5,527° C. The most intense heat we can produce is that developed in the electric arc discharge, and an eminent French chemist and metallurgist, M. Moissan, by employing special arrangements and very powerful currents, has thus been able to obtain in his “electric furnace” a temperature estimated at 3,500° C., which nearly approaches the lower of the above-mentioned limits, and he has thereby produced many new and unexpected chemical combinations of refractory elements. Among the most striking of his results is the formation artificially of real crystalline diamonds. He found that carbon is freely dissolved by several of the metals in fusion at the temperature of the electric furnace. When the carbon separated from the metals, as they cooled and became solid, it was always in the condition of graphite. The carbons of the electric poles were readily attacked by molten iron, and it was from the solution of carbon in iron that Moissan prepared his diamonds. The fact of carbon thus combining with iron was of course no discovery, as the reader already knows; and the resulting combination was found, on allowing the metal to cool, to be simply cast iron, the greater part of the carbon separating out in the graphitic form. But M. Moissan, having studied the conditions of the Kimberley mines, and recognizing the probability of the diamonds having taken their origin at very great depths, where the pressure due to the weight of superincumbent strata would be immense, was struck with the idea of pressure being in some way a factor in their formation; and it occurred to him that the carbon might separate from its liquid condition in the iron in the crystalline, and not in the graphitic form, if the solidification could be effected under great pressure. The apparently insurmountable difficulty of applying an enormous pressure to a small quantity of molten iron (half a pound) yielded to the experimenter’s ingenuity. He took advantage of the circumstance that cast iron at the moment of solidification expands, a property upon which depends its use for many purposes. If then the fused mass were suddenly cooled on the outside, we should have a shell of solid iron enclosing a nucleus of still fluid metal, which, on cooling in its turn, would tend to expand, and by so doing would exert a great pressure within the shell by which it was confined. At first Moissan plunged his glowing crucible into cold water, but a method of more rapidly cooling it was to immerse it in melted lead. It seems a strange proceeding to cool the crucible by surrounding it with hot metal, yet the difference of the temperatures was sufficient to produce the desired effect, the cooling contact of water not really operating on the intensely heated body, which becomes separated from the liquid by a coating of steam. When the mass of iron was dissolved off, diamonds of all kinds were found in the residue, and, though extremely small, some crystals were perfect in shape and colour; every variety that occurred in the mines being found reproduced in tiny size. There was also some graphite in the residue. Many more crystals of “pure water” were obtained by the lead-cooling than by the water-cooling, as the former process gave some flawless cubes and octahedra. The largest of the set was only 1/50 inch across, and although of perfect form when first extracted, within the course of three months it had spontaneously split up into fragments.

There was evidently no danger of M. Moissan’s manufacture of diamonds from coke causing consternation at Kimberley; though it would not be without interest to speculate upon the consequences had the French savant achieved the greater triumph of turning out carbon crystals in every respect equal to the productions of nature’s own laboratories. What a drop there would have been in the shares of the De Beers Mines Consolidated! What heaviness of heart would have fallen upon those great ladies who exult in the exclusive possession of priceless tiaras and precious necklaces flashing with the resplendent gems! From a scientific point of view, M. Moissan’s fabrication of even those minute crystals, which so soon spontaneously crumbled into fragments, is a distinct and valuable success; for, notwithstanding their diminutive size and instability, they show us that art has so far succeeded in imitating the processes of nature, that some of her secrets have been revealed. Though we know the exact chemical composition of all kinds of crystallized minerals, very very few of these have we been able to imitate artificially. Nor is this to be wondered at; for nature’s resources are immense compared with ours: she can command temperatures unlimited by which to form her solutions or liquefactions; prodigious pressures to keep them close; and time immeasurable—geological time—in which to let them cool, and their particles freely coalesce into geometric forms. Human agency, being obviously unable to reproduce, even on the smallest scale, such conditions as attended the deposition and slow cooling of the earth’s crust, may not hope to rival the products of the planet’s prime. So the fair owners of the earth-born gems may possess their souls in peace, free from any fear of the chemists’ crucibles; and the Kimberley Diamond Companies are not likely to suffer panics from the results of scientific researches, and probably will continue to pay their handsome dividends for time indefinite.

But curiously enough, a discovery of the latest years of our century has revealed the existence of diamonds in a region not mapped by the most advanced of geographers—a region which indeed cannot be defined by degrees of latitude and longitude. In the recesses of an unquestionable meteorite—one of those celestial lumps of iron of which mention has been made in the earlier pages of this volume—real diamonds have been found. These quite resembled the products of M. Moissan’s experiments, being extremely small, but including clear and perfectly shaped crystals, associated with black ones, and also with much graphite in more or less definite forms. So very limited, however, could be the quantity of diamonds obtainable from this hitherto unsuspected source, that even if they rivalled in quality the finest stones from the South African mines, it might be difficult to form a “Company” for their exploitation. Still, there is the possibility of some one falling in with a little meteorite containing some mature full-sized carbon crystals, and such a one might be considered equally fortunate with the finder of the famous Australian nugget “Welcome” (£25,000). The association of diamonds with the ferruginous matter of the “blue ground” in the Kimberley pipes, their crystallization out of iron in M. Moissan’s experiments, and their presence in iron meteorites, would seem to point to special relations between the two elements, iron and carbon. Some of these relations are exemplified in another way by the profound modification effected in the physical properties of iron, by its combination with a very small quantity of carbon, as in some kinds of steel; or again, by the differences between white cast iron and grey cast iron, as determined by the condition of the carbon in each.

NEW METALS.

The chemistry of the nineteenth century can boast of a series of discoveries more brilliant and more numerous than ever belonged to any other science within a like period. And the advantage to the world must have been great, for chemistry more directly than any other branch of knowledge ministers to the useful arts and promotes the comfort and well-being of society. The science itself, as it now exists, is almost the creation of the present age. But its recent developments cannot be here discussed; nor, of the immense number of new products with which it has enriched the world, can more than a very few be brought under the reader’s notice in the remaining pages of the present work. Among the most striking of the remarkable series of discoveries by which Sir Humphrey Davy penetrated the mysteries of matter was the isolation of the alkali metals—a circumstance which marks an important era in the history of chemistry. That the alkalies were oxides of unknown metals had indeed been previously surmised by chemists, from the fact of their behaving like metallic oxides in neutralizing and combining with acids to form the class of compounds called salts. All attempts to decompose these alkalies had proved fruitless until Davy separated the metal potassium from potash, in 1807. When, however, this alkali had once been proved a compound, more correct ideas were introduced into chemical science; the nature of other alkalies and earths was explained in like manner, and new and powerful re-agents were placed in the hands of the chemist.

Davy first obtained potassium by exposing to the action of the voltaic current a fragment of potash which had become moist on the surface by exposure to the air. The battery was formed of the then unprecedented combination of two hundred pairs of 6–inch plates on Wollaston’s plan, which was constructed for the Royal Institution of London. The heat produced by the passage of the current fused the potash, and globules of metallic potassium were separated at the negative wire. This method yielded the metal in very small quantities only, and at a great cost. Gay Lussac and Thenard soon afterwards found that potassium could be obtained more cheaply and in greater abundance when fused potash was made to flow over iron-turnings heated to whiteness in a gun-barrel, and the hydrogen and potassium vapour were passed into a cooled receiver, in which the latter body was condensed. The metal is now obtained by heating potassium carbonate with charcoal. For this purpose it suffices to heat crude tartar in a covered vessel from which air is excluded. The tartar is first calcined in a crucible until all combustible vapour has been driven off. The charred mass, which now consists of potassium carbonate mixed with finely-divided carbon, is then broken into lumps and quickly introduced into a wrought-iron retort, which is heated in a furnace to nearly a white heat. A receiver in the form of a flat iron box, 12 in. long, 5 in. wide, and ¼ in. deep, is adapted to the neck of the retort, and is kept cooled by the application of a wet cloth on the outside. The potassium thus obtained is not pure, and it must be distilled in an iron retort, as otherwise a powerfully detonating compound is apt to be formed by a portion of the metal combining with carbonic oxide.

Immediately after his discovery of potassium Davy obtained sodium in the same manner, and Gay Lussac and Thenard also procured it by the same process they used for the sister metal. Sodium is now extracted on the manufacturing scale for use as an agent in the reduction of two other metals, of which we shall have to speak. A mixture of dried sodium carbonate, powdered charcoal, and chalk is heated in wrought-iron cylinders, about 4 ft. long, 5 in. internal diameter, and ½ in. thick. The chalk takes no part in the chemical action, but is added in order to give the sodium carbonate when it fuses a pasty consistence, and thus prevent the separation of the charcoal. A number of these iron cylinders are set in a reverberatory furnace; but they are coated with fire-clay and enclosed in earthenware tubes, to prevent their destruction by the intense heat. To one end of each cylinder a receiver is adapted, of the form and dimensions already described for potassium. The other extremity is closed by an iron plug, luted with fire-clay. When the charge in a cylinder is exhausted, a fresh one is introduced by removing the plug, taking out the residue, and inserting a new supply of the mixture made up in a canvas bag. The operation is therefore continuous, and the metal obtained is nearly pure, as sodium does not exhibit the same tendency as potassium to form compounds with carbonic oxide.

Potassium and sodium are extremely soft metals; they are lighter than water, upon which they float, at the same time rapidly decomposing that compound by displacing half the hydrogen, which is set on fire by the heat. The instant a piece of potassium touches the surface of water, a violet flame bursts forth; but with sodium no flame appears unless the metal is dropped on warm water, or prevented from swimming about. Since these metals are thus capable of displacing hydrogen from its combination with oxygen at ordinary temperatures, it follows that they must have a powerful affinity for oxygen; and, indeed, they can only be preserved in rock oil, for they rapidly combine with the oxygen of the air. The great attraction of these metals for oxygen, and for chlorine and other similar bodies, induces the chemist to employ them for separating such bodies from their combination with other metals. Sodium is generally employed for this purpose, as being far cheaper than potassium.

Among the sixty-nine elementary or undecomposable substances which, variously combined, constitute the whole material of our planet, so far as we are acquainted with it, no fewer than fifty-six are metals. Of these fifty-six metals very few are found in a free or uncombined state, like the gold described in the last article. On the contrary, the whole of the metallic elements of the globe, with insignificant exceptions, exist in nature in a state of combination with one or more of the other thirteen non-metallic substances. In this condition they form the stony masses which are termed the ores of the more common metals, and they constitute also the earths, the metallic bases of which were, until recent times, unsuspected and unknown. Davy followed up his discovery of the metals of potash and soda by experimental demonstrations that the earths alumina, magnesia, and others, were really oxides of metals; and when the nature of these substances had once been established, chemists soon devised means for readily obtaining their metallic bases in an isolated form. The new metals which have been thus isolated all deserve the attention of the chemist; and the general reader will probably also regard with interest the processes by which two of these new metals, for which practical applications have been found, are extracted, and the properties which have caused them to be produced on the commercial scale. These are aluminium, the metallic base of common clay; and magnesium, the metallic base of common magnesia, and Epsom salts, and a constituent of dolomite, or magnesian limestone.

Aluminium was first isolated by Œrsted, in 1827, by decomposing its chloride by means of potassium. The chlorine leaves the aluminium to combine with the potassium, and thus the former is set free. Wöhler effected some improvements in Œrsted’s process, and he first obtained the metal in malleable globules. It is, however, to Deville that we are indebted for the invention, in 1854, of a process which admitted of application on a manufacturing scale. He obtains chloride of aluminium by mixing alumina (the oxide of the metal) with powdered charcoal made into a paste with oil, and heating the mixture in a tubular earthenware retort, like those sometimes used in the manufacture of coal-gas, while a current of dry chlorine is made to pass through the vessel. The charcoal combines with the oxygen, forming carbonic oxide, a permanent invisible gas; and the aluminium unites with the chlorine, giving rise to aluminium chloride, which, being volatile, sublimes into a chamber lined with glazed tiles, in which it condenses as a yellow translucent mass. The metal is reduced from the chloride in the following manner: A tube of hard glass, about an inch and a half in diameter, is placed over a furnace, or chaffing-dish, as shown in Fig. 336, where D C is the tube, and G G an iron pan for containing the red-hot charcoal. Into the part of this tube marked E, about half a pound of dry aluminium chloride has previously been introduced, and is kept in its place by plugs of asbestos. A current of dry hydrogen gas, perfectly free from air, is passed through the tube; the gas being generated in the vessel, A, and in B passed over some substance which removes from it all moisture. The aluminium chloride is then gently heated by placing red-hot charcoal beneath it, so that any hydrochloric acid it may contain may be expelled. A long narrow porcelain tray, or “boat,” containing pieces of sodium, F, is then introduced into the tube; and, the current of hydrogen being still maintained, heat is applied to the part of the tube containing the sodium, and the aluminium chloride is made to distil over by a regulated heat. As it passes over the sodium, it is reduced with a vivid glow. The aluminium is set free, and collects in the tray with the double chloride of sodium and aluminium which is produced by the reaction. The tray is removed and more strongly heated in a porcelain tube through which a current of hydrogen is passing, and the metal is thus obtained in globules.

Messrs. Bell, of Newcastle, undertook the manufacture of aluminium by a system founded on this process. The first step is the preparation of pure alumina, which may be obtained by igniting ammonia alum, or by precipitating from a solution of alum free from iron, or from sodium aluminate made from the mineral called bauxite. The precipitate of hydrated alumina, mixed with charcoal and common salt, is made into balls and dried. These balls, which are about as large as an orange, are placed in upright earthenware retorts, which are heated to redness, while a current of dry chlorine is passed through them. The volatile double chloride of aluminium and sodium distils over, and is condensed in chambers lined with earthenware. This substance is mixed with powdered fluor-spar, or with cryolite (itself a compound of aluminium), which serves as a flux; and small pieces of sodium are interspersed throughout the mixture. The proportions are ten parts of the double chloride, five of fluor-spar, and two of sodium. This mixture is thrown upon the hearth of a reverberatory furnace, and the doors are shut to exclude air. A very intense action occurs: the chlorine, quitting the aluminium, seizes on the sodium, and their combination is attended by an enormous increase of temperature. The fused aluminium is run off from the furnace together with the slags which are produced by the operation. In this way, with a furnace having a hearth 16 ft. square, about 16 lbs. of aluminium can be obtained in one operation.

Rose, the eminent German metallurgist, prefers to obtain aluminium from cryolite, which is a compound of sodium, aluminium, and fluorine, found in large quantities in Greenland. It is powdered and mixed with common salt, and with the mixture a certain quantity of sodium cut into small pieces is uniformly mingled. The whole is thrown into a heated crucible, previously lined with a fused mixture of cryolite and salt, and more of the same mixture is poured upon the contents of the crucible, which is then covered and exposed to a red heat for two hours. The aluminium generally collects into buttons, which may be easily melted together by heating them in a crucible with common salt.

It will be obvious, from the preceding account of the processes of extracting aluminium, that the cost of the metal must depend upon that of sodium; and the same remark will apply to the case of magnesium. It is interesting to observe how the price of the alkaline metals has decreased as improved processes have been devised, and as the scale of production has increased with the commercial demand for the article. Prepared by Gay Lussac and Thenard’s process, these metals were produced in but small quantities, and were sold at £5 per oz. When the mode of reducing them by charcoal came into operation, the price fell to 30_s._ per oz.; and the researches of Deville so far improved the processes, that in 1854 sodium could be procured for 5_s._ per oz. Mr. Gerhard, of Battersea, subsequently manufactured sodium, so that it can now be retailed at less than 1_s._ per oz. The price of aluminium before Deville’s investigations was about 24_s._ per oz., but now the metal can be purchased at about one-eighth of that cost. [1875.]

Aluminium is a white malleable metal, in colour and hardness not unlike zinc. Its colour is not so white as that of silver, as it has a marked bluish tint. It can be rolled into very thin sheets, and by rolling it becomes harder and more elastic. It can also be drawn into fine wire. It is remarkably sonorous, and a suspended bar gives out a clear musical note when struck. Perhaps no property of aluminium more strikes a person, who examines the metal for the first time, than its lightness. It is, in fact, only two and a half times as heavy as water, while zinc is seven times, silver ten and a half times, and gold more than nineteen times as heavy as water. It retains its lustre in dry or in moist air for any length of time, and at all ordinary temperatures. It is not acted upon by nitric or sulphuric acids, but is attacked by hydrochloric acids and by alkaline solutions with great energy. It has great rigidity and tenacity, and can be turned, chased, and filed with the greatest ease, and without clogging the tools. In the Paris Exhibition,[15] M. Christofle showed spoons and forks and a cup made from it; and it may be mentioned, as showing the hardness and strength of the metal, that the cup could be allowed to fall on a stone pavement without being indented. The metal gives a good impression by casting; and by striking under a die, some admirable medals have been produced in it. Aluminium has hitherto been chiefly used for ornamental articles, and for purposes where lightness and rigidity are desirable, such as in the tubes of telescopes, opera glasses, beams of balances, &c. Its unalterability and admirable working qualities have also caused it to be used for cheap trinkets and ornaments—such as watch-cases, bracelets, combs, seals, penholders, candlesticks, &c. It is, however, incapable of receiving the lustrous polish of silver, as it has a decidedly blue tint, so that it will probably never replace silver for ornamental plate; but it would be a good material for egg and mustard-spoons, as it is quite unaffected by the sulphur compounds which so readily tarnish silver. It has been suggested that if aluminium could be procured cheaply enough, “its hardness, lightness, and incapability of rusting would render it admirably adapted for the helmets and cuirasses of the cavalry; it would make splendid field-guns, as strong as the present ones, and not one-third of their weight; and, in sheets, it might serve as an incorrodible roofing, far lighter and more durable than zinc. It would admirably replace copper, if not silver, for the purpose of coinage. A crown-piece in aluminium would hardly weigh more than a shilling in silver, or a piece the size of a penny about as much as a copper farthing. The same qualities of lightness, hardness, and incorrodibility also excellently fit it for the beams of delicate balances, and for the small weights used by the analytical chemist. It would make admirable utensils for the more delicate operations of cooking—replacing the copper ones, which render pickles and soups so poisonous. It is extremely sonorous, and would make capital bells.”

Footnote 15:

Of 1867.

Some difficulty in working the metal has occurred from the want of any suitable solder. This difficulty has been overcome by electrolytically coating the metal with copper at the place where it has to be united with others, and then soldering the copper in the ordinary manner. Aluminium readily forms alloys with copper, silver, and iron. The alloys with copper vary in colour from white to golden yellow, according to the proportion of the metals. Some of these alloys are very hard and possess excellent working qualities. The alloy of copper with 10 per cent. of aluminium, which is called aluminium bronze, has been manufactured by Messrs. Bell in considerable quantities. It is made by melting a quantity of very pure copper in a plumbago crucible, and when the crucible has been removed from the furnace, the solid aluminium is dropped in. An extraordinary increase of temperature then occurs: the whole mass becomes white hot, and unless the crucible be made of a highly refractory material, it is fused by the heat developed in the combination of the two metals, although it may have stood the heat necessary for the fusion of copper.

The qualities of aluminium bronze have been investigated by Lieut.-Col. Strange, who finds that the alloy possesses a very high degree of tensile strength, and also great power of resisting compression, its rigidity, or power of resisting cross strains, is also very great; in other words, a bar of the alloy, fixed at one end and acted on at the other by a transverse force tending to bend it, offers great resistance,—namely, three times as much as gun-metal. An advantage attending the use of the alloy for many delicate purposes is found in its small expansibility by heat; it is therefore well adapted for all finely-graduated instruments. It is very malleable, has excellent sounding properties, and resists the action of the atmosphere. It works admirably with cutting tools, turns well in the lathe, and does not clog the files or other tools. It is readily made into tubes, or wires, or other desired forms. The elasticity it possesses is very remarkable; for wires made of it are found to answer better for Foucault’s pendulum experiment than even those of steel. These admirable qualities would seem to recommend the alloy for many applications in which it might be expected to excel other metals. It appears, however, that the demand for it has not met the expectations of the manufacturers, and the production has been somewhat diminished of late, although it is used to some extent for chains, pencil-cases, toothpicks, and other trinkets. When more than 10 per cent. of aluminium is added to the copper, the alloy produced is weaker; and if the proportion is increased beyond a certain extent, the bronze becomes so brittle that it may be pulverized in a mortar.

The metal magnesium was first prepared, in 1830, by the French chemist Bussy, by a process similar to that by which Deville obtained aluminium. Bussy heated anhydrous magnesium chloride with potassium in a porcelain crucible; and when the vessel had cooled, and the soluble residue had been dissolved out by water, the metal was found as a grey powder, which could be melted into globules. The recognition of the metal as the base of magnesia is, however, due to Davy. About a quarter of a century after Bussy’s discovery Deville having shown that sodium could be substituted for potassium in such reductions, the metal became more cheaply producible, and soon afterwards Bunsen and Roscoe pointed out its value as a source of light. Mr. Sonstadt devoted himself to the elaboration of a method of working Deville’s process on the large scale, and he succeeded in establishing a company in Manchester for the manufacture. The process as carried on at the company’s works in Salford is thus described in the “Mechanics’ Magazine,” 30th August, 1867:

“Lumps of rock magnesia (magnesium carbonate) are placed in large jars, into which hydrochloric acid in aqueous solution is poured. Chemical action at once ensues: the chlorine and the magnesium embrace, and the oxygen and carbon pass off in the form of carbonic acid. The result is magnesium in combination with chlorine, and the problem now is how to dissolve this new alliance—to get rid of the chlorine and so obtain the magnesium. First, the water must be evaporated, which would be easy enough if not attended with a peculiar danger. To get the magnesium chloride perfectly dry it is necessary to bring it to a red heat; but this would result in the metal dropping its novel acquaintance with chlorine and resuming its ancient union with oxygen. To avert this re-combination, the magnesium chloride whilst yet in solution is mixed with sodium chloride (i.e., common salt), and thus fortified, the aggressions of oxygen whilst drying are kept off. The mixture is exposed in broad open pans over stoves, and when sufficiently dry, the double salt is scraped together and placed in an iron crucible, in which it is heated until melted, whereby the last traces of water are driven off. It is then stowed away until required in air-tight vessels, to prevent deliquescence. Here comes in that curious metal, sodium, also discovered by Davy. Five parts of the mixed magnesium and sodium chlorides, mingled with one part of sodium, are placed in a strong iron crucible with a closely-fitting lid, which is then screwed down. The crucible is heated to redness in a furnace, and its contents being fused, the sodium takes the chlorine from the magnesium. When the crucible has been lifted from the fire and allowed to cool, the lid is removed and a solid mass is discovered, which, when tumbled out and broken up, reveals magnesium in nuggets of various sizes and shapes, bright as silver.”

The crude metal also presents itself in the crucible as small grains, and even as a black powder. The whole is carefully separated from the refuse; it is purified by distillation in a current of hydrogen gas; and it is afterwards melted and cast into ingots. Magnesium is a very light metal, its specific gravity being only 1·743; that is, it is only one and three-quarter times heavier than water. When heated in the air it takes fire, and is rapidly converted into the oxide, magnesia. In the form of wire or of narrow ribbon, it burns easily in the air, producing a light of dazzling brilliancy, which among artificial modes of illumination is rivalled only by the electric light. This is the chief use at present made of the metal. Lamps have been contrived for burning the wire in such a manner as to obtain a steady light, the wire being pushed forward at a regulated rate by clockwork. The magnesium light is rich in the rays which act upon sensitive photographic plates, and it has been successfully employed in obtaining photographs of dark interiors, such as vaults or caverns, and for the exploration of mines and other dark places. The brilliancy of the firework displays which can be produced by magnesium far surpasses that obtainable by any other material used by the pyrotechnist. In such exhibitions balloons are sent up having burning magnesium attached to them; and the metal in the state of filings is also mixed with other materials. But magnesium is still a very costly metal, and while the firework-makers find it too expensive for common use, they complain that its brilliancy in occasional displays dulls by contrast the effect of the ordinary fireworks, with which the spectators are no longer satisfied.

Magnesium wire is not produced by drawing, as the metal is not ductile. The wire is formed by a method identical with that used in the fabrication of the leaden rope for making bullets (p. 330); that is to say, the metal is forced in a heated and softened state through a small opening in an iron cylinder. The intensity of the magnesium light has been measured by Bunsen and Roscoe. They say that 72 grains of magnesium, when properly burnt, evolve as much light as 74 stearine candles burning for ten hours, and consuming 20 lbs. of stearine. Lamps in which magnesium may be steadily burnt are made by Mr. F. W. Hart, of London. In the more elaborate forms of these lamps, there are springs and wheels for pushing forward the magnesium ribbon, or a strand of magnesium wire, into the flame of a spirit-lamp; while at the same time the magnesium wire is made to revolve on its axis, in order to overcome its tendency to bend down, which would be a great disadvantage when the light is used for optical apparatus. But for ordinary purposes a much simpler arrangement suffices: the magnesium ribbon or wire is coiled on a drum, from which it is drawn off by passing between two little rollers, which are turned by hand. The wire or ribbon is drawn off the drum by the rollers, and pushed forward through a guiding tube, which brings it into the apex of the flame of a spirit-lamp. In this simpler form of lamp the rate is, of course, directly dependent on the person who turns the winch of the feeding-rollers; but in the automatic lamp there are appliances for adjusting the rate; the suitable speed must be first found by trial, and then the apparatus is to be regulated accordingly. By means of these lamps photographs can be taken as quickly as with sunlight, on account of the abundance of chemically-active rays given out by the burning magnesium. It has been found that an equivalent of magnesium, in combining with oxygen, liberates a larger amount of heat than the equivalent quantity of any other metal, not excluding even potassium. Magnesium forms alloys with several other metals, such as lead, tin, mercury, gold, silver, platinum. All these alloys are brittle, and have a granular or crystalline fracture. They are too readily acted on by air and moisture to be of any service in the arts. The alloy of 85 parts of tin with 15 of magnesium is hard and brittle; its colour is lavender, although both constituents are white, or nearly so; and it decomposes water at ordinary temperatures. Both metallic magnesium and aluminium furnish useful re-agents to the scientific chemist. The latter metal, when fused, dissolves boron, silicon, and titanium, and on cooling deposits these elements in the crystalline form, this being the only known process for artificially preparing them in the crystalline state.

Since the above paragraphs were written, the price of sodium has been further greatly reduced, and it can now (1890) be purchased in bulk at about 4_s._ per lb. This cheapness has brought the substance into use for the reduction of other metals and one consequence has been a great fall in the price of aluminium. At Salindres, in France, the process of obtaining this metal that has been described on page 587, has been in use for many years, during which considerable quantities of aluminium have been produced, the output for 1882 being stated as 5,280 lbs. Aluminium has lately been prepared by a company at Wallsend-on-Tyne from cryolite, a mineral which is found only in Greenland, but occurs there in great abundance. Cryolite is a double fluoride of aluminium and sodium, and the processes for its reduction consist in fusing it with common salt in a reverberatory furnace, drawing off the mixture into an iron vessel, and stirring into the fused mass a certain quantity of sodium. This produces a violent action, on the cessation of which the slag is poured off, and the metallic aluminium is found as a “button” at the bottom of the converter. For obtaining a purer metal, the fusion is made in crucibles, and the sodium is added in two operations without removing the crucible. The yield of aluminium is about 8 per cent. of the weight of cryolite, and three parts of sodium are required to furnish one part of aluminium. Another large manufactory of aluminium is in operation at Oldbury, near Birmingham. There is a special difficulty in the metallurgy of aluminium, arising from the fact of the qualities of the metal being much deteriorated by the presence of a very small amount of foreign matters such as iron, silicon, &c., at the same time that no process has been found for purifying the product from these substances. If the aluminium is to be pure it must be so prepared at the first. Electrolysis has been proposed as a means of reducing the compounds, and obtaining the metal free from admixtures. Experiments seem to show that the dynamo-electric machine may be applied to this purpose, as well as to the reduction of sodium compounds, when certain practical difficulties arising from the chemical energies of the liberated substances have been overcome. What is called the “electric” furnace has been successfully used in the production of aluminium bronze. It is a rectangular iron box, 5 feet long, 1 foot deep, and 15 inches wide, with electrodes formed of rods of carbon 30 inches long and 3 inches in diameter. It is charged with a mixture of 25 parts of corundum (native crystallized oxide of aluminium), 12 parts of carbon, and 50 parts of granulated copper. This is covered at the top by lumps of charcoal, and a lid is fastened over the whole. The current from a powerful dynamo is sent through the carbons, and in about ten minutes the copper is melted, when the electrodes, at first only a few inches apart, are moved to an increased distance, and the strength of current increased. The corundum is reduced, the aluminium alloying itself with copper, and the oxygen combining with the carbon to form carbon monoxide, which is driven off. The resulting alloy is cast into ingots, its percentage of aluminium ascertained, and then it is melted with enough copper to produce aluminium bronze (page 719). The price of aluminium, which was as already stated about 3_s._ per ounce in 1875, has been so much reduced that the metal may now (1890) be purchased for 11_s._ or 12_s._ per lb. We may therefore expect to see wider applications of its excellent qualities. Though the price per lb. is still much higher than that of copper-–22 or more times as much—the metal is so much lighter that a lb. of aluminium occupies nearly 3⅓ times the space of a lb. of copper, so that, taking bulk for bulk, aluminium is only about seven times as dear as copper. [1890.]

When first introduced by Deville, in 1854, aluminium cost £20 per lb.; but its prospective value for application in the arts was recognised, and in two or three years afterwards it was put on the market at 40_s._ per lb. It was then, as already remarked, applied to many purposes where lightness is desirable, such as for the tubes of telescopes, opera-glasses, the mounting of photographic lenses, &c. And in 1888, when the production of sodium had been cheapened and applied to the separation of aluminium, the price of the latter metal fell to 18_s._ per lb. In the meantime, the cheap electricity of the dynamo caused attention to be again directed to the original electrolytic method; but many difficulties in detail had to be overcome in applying this process on the commercial scale. At length the sodium process was superseded; and by the beginning of 1890, a Swiss company was producing aluminium at 11_s._ per lb. In the course of the following year they succeeded in bringing the price down to 2_s._ per lb.; and again three years later, namely at the beginning of 1894, they could offer the metal at 1_s._ 7_d._ per lb. The conditions required for effecting this great reduction were found in driving the dynamo machinery by water-power, and in an abundant supply of cryolite at moderate cost. This cheapness of production at once placed the Swiss company in the position of being the largest and most successful aluminium manufacturers in the world, so that in 1892 they had realised a net profit of £21,563, paying their shareholders 8 per cent., and, further, in 1893, the net profit was half as much again, and the dividend was increased to 10 per cent. A British aluminium company has recently been formed in London for acquiring the rights of working all the processes of the successful Swiss company, purchasing outstanding English patents, amalgamating with certain existing companies, and for working the bauxite deposits in Ireland, &c., &c. There is every reason to believe that an important result of this enterprise will be a still further reduction in the price of this metal, and consequently a great extension of its applications. And now (September, 1895) we have already heard of a further reduction in the price of this metal, which, at the present time, can be purchased in bulk for about 1_s._ 6_d._ per lb.

INDIAN-RUBBER AND GUTTA-PERCHA.

INDIAN-RUBBER.

Researches into the history of the human race in remote ages have revealed the fact, that before man knew how to extract metals from their ores, his only implements were formed of stone; and before he became acquainted with iron, there was an intermediate period in which the more easily obtained metal, copper, had to serve as the material for all tools and weapons. Hence archæologists speak of the stone age, the bronze age, and the age of iron. If we were obliged to name the nineteenth century after the material which distinctively serves in it for the most extensive and varied uses, surely we should call it the Age of Indian-rubber!

The industrial application of Indian-rubber is entirely modern. The substance itself appears, however, to have been known to the natives of Peru from time immemorial, and to have been used for the preparation of some kind of garments. Although the first specimens were sent to Europe so long ago as 1736, and the substance was from that time submitted to many investigations, no other use was found for it up to the year 1820 than to efface from paper the marks made by pencils. From this it derives the name by which it is commonly known. It has also been called “gum elastic,” and caoutchouc from the Indian name. Crude caoutchouc is the product obtained by the spontaneous solidifying of the milky juice of certain tropical plants—such as the Hævea elastica, Jatropha elastica, and the Siphonia cautshu. The first grows chiefly in South America, and in the basin of the Amazon forms immense forests. At a certain season each year bands of persons, called “seringarios,” armed with hatchets, visit these forests for the purpose of extracting the caoutchouc. They make incisions into the trunk, and the milky juice immediately runs out, and drops into a vessel placed to receive it, and attached to the tree by means of a lump of clay. In about three hours the juice ceases to flow, and the seringario collects the products of the incisions in one large vessel. By dipping a board into this vessel, it becomes covered with the juice; and when this is allowed to dry, the caoutchouc remains as a thin brownish yellow layer. The caoutchouc is not dissolved in the juice, but is merely suspended in it; and to hasten the drying and coagulation of the liquid, the board is warmed over a smoky fire made with green wood. When alternate immersions and drying have covered the board with a sufficient thickness of caoutchouc, the layer is slit open with a knife, and the board is withdrawn. This is the best kind of crude caoutchouc, because it is free from all admixture of foreign bodies except the carbon derived from the smoky flame. The bottle Indian-rubber is moulded on pear-shaped lumps of clay, which are covered with successive layers of the milky juice; when a sufficient thickness has been attained, the clay is removed by soaking in water.

Up to 1820, as already mentioned, Indian-rubber was used only for effacing pencil-marks, and about that time a piece half an inch square sold for two shillings and sixpence. But the extreme elasticity and extensibility of this singular substance was attracting the attention of practical men in England, Scotland, and France. One of the earliest patents obtained in this country for applications of caoutchouc was taken out by Mr. Thomas Hancock, of Newington, in 1820. This gentleman has written an account of the Indian-rubber manufacture from the commencement, and the book is extremely interesting from the clear and simple manner in which the inventor describes how he effected one improvement after another in his processes and machinery. Mr. Hancock had, previous to his turning his attention to Indian-rubber, no acquaintance with chemistry; but he was skilled in mechanical engineering and the use of tools, and this knowledge proved to be precisely the kind most valuable for dealing with the first stages of caoutchouc manufacture. His first patent was for the use of Indian-rubber for the wrists of gloves, for braces, for garters, for boots and shoes instead of laces, and for other similar purposes. The rings for the wrists of gloves, &c., were simply cut from the bottle Indian-rubber by machinery the patentee himself contrived for that purpose. Mr. Hancock next arranged an apparatus for flattening the raw Indian-rubber by warmth and pressure, so as to make it available for the soles of boots, &c. He relates the practical difficulties he had to encounter in his operations, and the manner in which he overcame them. He soon noticed and utilized the fact that two clean freshly-cut surfaces of caoutchouc, when pressed together, cohere and unite perfectly. This further led him to devising a machine by which all the waste cuttings and parings might be worked up. This machine consisted of a cylinder revolving within a cover, both being provided with steel teeth, by which the pieces of caoutchouc placed between them were torn into shreds, and then kneaded into a solid coherent mass of homogeneous texture. The first machine of this kind made by Mr. Hancock would work up about 1 lb. of Indian-rubber; but now machines on the same principle are in use operating on more than 200 lbs. of material at once, and turning it out on a roll 6 ft. long, and 10 in. or 12 in. in diameter.

While Hancock was thus successful in mechanically working Indian-rubber, Macintosh, of Glasgow, found means of effecting its solution by coal-naphtha, and he obtained, in 1823, a patent for the application of his discovery to the fabrication of waterproof garments. Waterproof cloth, or “Macintosh,” is prepared by varnishing one side of a suitable fabric with a solution of caoutchouc, or by covering one side of a cloth with a thin film, and then bringing it into contact with a second piece similarly prepared—the two caoutchouc layers becoming incorporated when the double cloth is passed between rollers. Other solvents for Indian-rubber have been discovered in ether, chloroform, sulphide of carbon, and rectified turpentine. By treatment with these liquids it swells up, and eventually dissolves, producing a viscid ropy mass, which, by evaporation of the solvent, leaves the caoutchouc with all its original elasticity. By the use of these last-named solvents the persistent and disagreeable odour occasioned by coal-oil is avoided. Mr. Hancock relates that when the manufacturers had overcome all obstacles, and had succeeded in producing thin, light, pliable, and perfectly waterproof fabrics, they had to encounter another quite unexpected difficulty—the tailors set their faces against the new material, and could not be induced to make it up! The manufacturers were, therefore, obliged themselves to fashion waterproof garments, and retail them to the public. This, however, turned out to be a benefit, for the seams were made waterproof, so as to exclude even the little water which would otherwise pass in by capillary attraction at the stitches.

It will now be observed that there are two distinct modes of working caoutchouc: by dealing, viz., with the solid material, or with the solutions. Thus, from a solid disc of caoutchouc long ribbons of the material may be cut, and these ribbons, by being passed between a set of circular knives, may be divided into a number of square threads. These threads may then be drawn out to six or ten times their length; and, if wound and maintained in this state of tension for forty-eight hours in a warm place, they will lose their condition of tension, and their elongated form will become their natural or unstrained one. In this manner are the Indian-rubber threads prepared, which, covered with silk or other material, form elastic fabrics such as those used in the sides of boots. The circumstance of caoutchouc, when heated for some hours at a temperature a little above the boiling-point of water, retaining whatever form it has during the heating, is the basis of methods of obtaining thin sheets and other forms of the material. Tubes are made by forcing the heated caoutchouc through an annular opening by application of great pressure; it sets in cooling, preserving a section corresponding with the orifice through which it issues. In another mode of forming tubes, a paste composed of caoutchouc, oxide of zinc, and lime, is formed into sheets, which are cut into strips. The strips are folded longitudinally, and the edges are cut together at an angle of 45° with the surface, so that the cut surfaces may meet each other when the strip is rolled on a mandril to give it a cylindrical form. A slight pressure suffices to solder together the cut surfaces, and the tube is then “vulcanized” by a process to be presently described.

The dissolved caoutchouc serves to prepare waterproof garments, round threads, sheets of Indian-rubber, &c. Fabrics are coated with the solution by pouring it on the material as it is passing horizontally from a roller. A straight-edge, under which the charged cloth passes, distributes the caoutchouc in a uniform layer, the thickness of which is regulated by the space between the knife-edge and the fabric. When sulphide of carbon is the solvent used, its evaporation is complete in about ten minutes, but with other solvents two or three hours are required. The caoutchouc is usually mixed with lampblack before being spread on the cloth, and the article is finished by giving the Indian-rubber layer a coat of gum-lac varnish. Sheets of Indian-rubber are obtained by spreading fifteen or twenty layers over a cloth, which is afterwards detached by wetting it with a solvent.

Threads of circular section are manufactured from a paste of caoutchouc, made by dissolving that substance in sulphide of carbon mixed with 8 per cent, of alcohol. This paste is placed in a cylinder, out of which it is forced by a piston through a number of circular holes, whence it issues in the form of filaments. These are received upon a stretched cloth, which moves along, carrying the parallel threads, until the sulphide of carbon has evaporated.

A modification of caoutchouc, possessing very valuable qualities for many purposes, was discovered by Mr. Charles Goodyear, and largely applied by him in the United States to the fabrication of waterproof boots. In 1842 these boots were imported into Europe, and it was seen that this form of the material had the advantages of not sticking to other bodies at any ordinary temperatures, and of preserving its elasticity even in the coldest weather, whereas ordinary Indian-rubber becomes rigid by cold. The cut edges of this variety of caoutchouc do not cohere by pressure. Mr. Goodyear attempted to keep his process a secret; but Mr. Hancock, having soon detected the presence of sulphur in the American preparations, set to work to discover how that substance was made to combine with the caoutchouc. He succeeded, and he obtained a patent for sulphurizing Indian-rubber before the original inventor had applied for one. Mr. Hancock found that a sheet of caoutchouc immersed in melted sulphur at 250° F., gradually absorbed from 12 to 15 per cent, of its weight of sulphur; and, further, that this does not in any way alter its properties. When, however, the sulphurated substance was for a few minutes exposed to a temperature of 300°, it acquired new qualities, which were precisely those of the modification employed by Mr. Goodyear for his impervious boots. This transformation effected by sulphur Mr. Hancock called vulcanization; and vulcanized Indian-rubber is now employed in nearly all the innumerable applications of caoutchouc, provided the presence of sulphur is not absolutely objectionable. Goodyear’s process consists in mixing the sulphur with the caoutchouc, the suitable proportion (7 to 10 per cent.) having been determined beforehand, and the sulphur ground up with the Indian-rubber in the masticating machine, or disseminated through the viscid liquid if a solution is used, or dissolved in the solvent employed. This gives better results than Hancock’s process, because the sulphurization is more uniform, and this method is therefore more largely employed. When the various articles have been fabricated in the ordinary manner from the mixture of caoutchouc and sulphur, they are enclosed in vessels, where they are submitted for two or three hours to the action of steam under a pressure of nearly 4 atmospheres, so that the steam may have a temperature of about 280° F. A still easier method, due to Mr. Parkes, consists in steeping the articles (which in this case should be thin) in a solution of one part of chloride of sulphur in sixty of bisulphide of carbon. The object becomes vulcanized by simple exposure to the air, without the aid of heat. But this process is said to be liable to cause the article afterwards to become brittle. The addition of oxide of zinc, carbonate of lead, and other substances, is found to yield a product better adapted for certain purposes than one in which only sulphur is used.

The list of applications of vulcanized Indian-rubber would be a very long one; but as a great number of these applications must be known to everybody, it will be unnecessary to specify them. It has lately been used for carriage-springs, for the tires of wheels, and for the rollers of mangles. Its employment in the construction of portable boats, pontoons, life-buoys, dresses for divers and for the preservation of life at sea, air-tight bags and cushions, air and water beds, cushions of billiard-tables, are a few of the thousand instances of its utility which might be quoted.

When the proportion of sulphur mixed with the caoutchouc is increased to 25 or 35 per cent., another product having qualities entirely different from those of vulcanized Indian-rubber is obtained when the mixture is heated. This is the jet-black substance termed ebonite or vulcanite, which is made into such articles as combs, paper-knives, buttons, canes, portions of ornamental furniture, and plates of electrical machines. It is in many cases an excellent substitute for horn and for whalebone, while for insulating supports, &c., in electric apparatus, it is unrivalled. It has a full black colour and takes a bright polish; and it may be cut, or filed, or moulded. It is very tough, hard, and durable. In the transformation of Indian-rubber into vulcanite, the temperature must be somewhat higher than that required for the production of the vulcanized Indian-rubber. The caoutchouc used is very carefully purified before it is incorporated with the sulphur; and the yellow paste formed by the mixture is subjected to the contact of steam at a temperature of about 310°.

GUTTA-PERCHA.

Gutta-percha is a substance very like Indian-rubber in its chemical properties, having the same composition, although in outward appearance very different. It was first sent to Europe in 1822, but did not become an article of commerce until 1844. It is the solidified juice of a tree (Isonandra percha) which abounds in Borneo and Malacca. The trunk of the tree grows to a diameter of 6 ft., but as timber it is valueless. When an incision is made through the bark and into the wood, a milky juice flows out, which speedily solidifies. Gutta-percha is a very tough substance, but is without the elasticity of Indian-rubber. It differs from the latter, too, in becoming softened by a gentle heat, and it will then readily take and retain any impressions with great sharpness and fidelity. Thus beautiful mouldings and other ornamental objects are easily made. It also has the valuable quality of welding when softened by heat. It is a non-conductor of electricity, and it is largely used for covering telegraph-wires, and especially for forming an insulating coating in submarine cables. It seems to have become known precisely at the time it was required for this purpose, and the success of ocean telegraphy is largely owing to its valuable properties. It is employed as a substitute for leather in soling shoes and boots, and in forming straps and bands for driving machinery; also in the preparation of tubes used for conveying liquids, and for speaking-tubes. Dilute mineral acids have no action upon it, and hence it is especially valuable for making bottles to contain hydrofluoric acid, which attacks glass. A drawback to the use of gutta-percha is its tendency to become oxidized when exposed to light and air, by which it entirely loses its power of becoming plastic by heat, and is converted into a brittle substance. But in the dark, or under water, it may, however, be preserved for an indefinite period without change.

Mr. Charles Hancock, in 1847, patented a machine for cutting the gutta-percha into slices. In this machine there is a circular iron plate, with three radial slots, in which knives are fixed somewhat in the manner of the cutting tool of a spokeshave. The lumps of gutta-percha drop against these knives as the plate is driven round, and the material is cut into slices, which have a thickness determined by the projection which has been given to the blades. Sometimes an upright chopper is used, with straight or curved blades. These slices are immersed in hot water, until they are softened, and they are then subjected to the action of rollers armed with toothed blades, called “breakers,” and also to the action of the mincing cylinder, which is furnished with radiating blades, and revolves partly immersed in the water. The material is carried out of the hot water to these machines by endless webs mounted on rollers. The breakers and mincing cylinders make about 800 revolutions per minute. The gutta-percha, thus reduced to fragments, is carried forward again by endless webs into cold water, where it is thoroughly washed and separated from the impurities, which fall to the bottom, while the lighter gutta-percha floats on the surface of the water.

Gutta-percha, like caoutchouc, can be combined with sulphur. The best product is obtained when a small proportion of sulphur is used along with some metallic sulphide. Mr. C. Hancock uses 48 parts of gutta-percha, 1 of sulphur, and 6 of antimony sulphide. These ingredients are thoroughly mixed and put into a boiler, where they are heated under pressure for an hour or two. Another method of treating gutta-percha was also devised by Mr. C. Hancock, who found that when this strange substance was exposed to nitric oxide gas (which is given off when nitric acid acts on copper) it became quite smooth, and acquired an almost metallic lustre, losing also all its stickiness. Another modification is formed by treating gutta-percha with chloride of zinc; and yet another by the action of a solvent, such as turpentine, a sulphide, sulphur, and carbonate of ammonia, employed simultaneously. Mr. Hancock mixes all these materials together in a “masticator,” and then applies heat to them while confined in a vessel under pressure. The product of these operations is a very singular modification of gutta-percha, in which the material assumes a spongy, elastic condition, and in this form it is used to form the stuffing of sofas, easy chairs, &c. Among the purposes to which gutta-percha has been applied besides the general one of waterproof tissues and fabrics, may be named the formation of straps, belts, bandages, cups, and other vessels, rollers for cotton-spinning machinery, hammers of pianofortes, cards for wool-carding, hammercloths, life-preservers, and trusses.

Gutta-percha is made into strips, bands, cords, or threads of any required section, by passing sheets of suitable thickness between rollers provided with grooves and cutting edges. For strips and bands the sheets are passed through the machine cold, and divided by the cutting-edges. But for round cords or threads sheets are supplied to the rollers from a receptacle in which they acquire a temperature of about 200° F. The material is forced to take the form of the grooves, the operation in this case being analogous to that of rolling iron bars. The gutta-percha cords are received as they issue from the rollers in a tank of cold water, from which they pass on, to be wound on reels or drums. It is obvious that cords of any section may be formed by making use of grooves of suitable shape. Tubes of gutta-percha are made by forcing the softened substance out of an annular orifice: it is received into vessels filled with cold water. Telegraph-wires are covered by a similar process—the copper wire being made to pass through the centre of a circular opening with the gutta-percha surrounding it. Picture-frames, &c., are made by forcibly pressing warm gutta-percha into the warmed moulds. Gutta-percha tubing is largely used everywhere for the speaking-tubes by which persons in remote apartments of even the largest building can converse. This is one of the labour-saving inventions of our day. It must have struck every one who has seen these speaking-tubes in operation in a large establishment, what a vast amount of running to and fro they save, and how much they expedite business by the convenient means they afford of giving orders and directions to persons in distant apartments. This tubing is also used for the conveyance of liquids, and it has been proposed to employ it instead of the ordinary leaden piping used for carrying water. It may seem to the reader that gutta-percha is too fragile a material to resist the pressure to which water-pipes are exposed. But, judging from some experiments made by the engineer of the Birmingham Waterworks, the power of gutta-percha tubing to resist pressure is quite extraordinary, and far beyond what would be supposed. The tubes experimented on had diameters of ¾ and ⅞th of an inch respectively. The water from the main, where the pressure was that caused by a head of 200ft., was in communication with these pipes for several weeks, and they were found unaltered in any way. In order to test the strength of the tubes, and find the greatest pressure they would bear, the engineer then had them connected with a hydraulic proving-pump; and here, when exposed to the highest pressure at which the ordinary water-pipes were tested, namely, to 250 lbs. on the square inch, they also remained intact. The pressure was afterwards increased to 337 lbs., but without any damage to the tubes.

The increasing importance of gutta-percha may be inferred from the continually augmented importation of the crude substance into this country. In 1850 only 11,000 cwt. were imported, but the quantity has increased year by year; and in 1872 we received nearly 46,000 cwt. The demand is still increasing; but there is reason to apprehend that under the stimulus of a rising market, the producers have collected the gutta-percha wastefully and with great destruction of the trees, so that it is not improbable that if the demand still increases, there may be a gutta-percha famine. The concreted juices of certain other trees have been proposed as substitutes for gutta-percha. None of these have as yet come into practical use. The increase in a few years of the quantity of Indian-rubber imported into the United Kingdom is perhaps more extraordinary. From the tables given in Mr. Hancock’s book, it appears that our imports of caoutchouc were 853,000 lbs. in 1850, but by 1855 they had amounted to 5,000,000 lbs.

ANÆSTHETICS.

The discovery which is indicated by the somewhat unfamiliar word[16] which heads this article is perhaps the greatest which has ever been made in connection with the science of medicine. At least, there is no other discovery of modern times which has so largely and directly contributed to the assuagement of human suffering. Nay, in this respect there is perhaps in the whole annals of the healing art no other which can rival it, if we except that famous one of Jenner’s which has arrested the ravages of small-pox. During the last thirty years, all the more formidable operations of the surgeon have been, in almost every case, performed with a happy unconsciousness on the part of the patient. In unconsciousness, induced by the same means, has relief also been found for severe suffering arising from other causes. The substances which are denoted by the word “anæsthetics” differ from the drugs which the older surgeons sometimes administered before an operation, in order to lull the patient’s sense of pain. They differ in their nature and in the mode of their administration; by the certainty and completeness of their action; by the entirely transient effects they produce, which pass off without leaving a trace.

Footnote 16:

From α (αν), privative, and αισθητικος, capable of perceiving or feeling.

To the great chemist whose name has already been mentioned as the discoverer of the metals of the alkalies and alkaline earths we are indebted for the first of the remarkable class of bodies we are about to discuss. The first work that Davy published had for its title “Researches, Chemical and Philosophical, chiefly concerning Nitrous Oxide and its Respiration.” This was in the year 1800, when the philosopher had hardly completed his twenty-first year. The work caused no little sensation in the scientific world, and it was in consequence of the reputation he acquired by these researches that Davy was appointed to the chemical professorship at the Royal Institution. Davy was not the original discoverer of nitrous oxide, but he first entered upon a full investigation of its properties, and announced the singular effect produced by its inhalation. The kind of transient intoxication and propensity to laughter which it excites have obtained for this compound the familiar name of laughing gas. Davy had by experiment on his own person proved the anæsthetic properties of this gas, for he had a tooth painlessly extracted when under its influence, and he says in the work above named that “as nitrous acid gas seems capable of destroying pain, it could probably be used with advantage in surgical operations where there is no effusion of blood.” Davy’s observations and suggestions were destined to lie barren for nearly half a century, but they nevertheless formed the basis of the great results which have since been attained.

Before proceeding farther, it will perhaps be well to make the unscientific reader acquainted with the chemistry of nitrous oxide. We may presume that he knows that atmospheric air is a mixture of the two invisible gases, nitrogen and oxygen (the small quantity of carbonic acid also present need not now be considered). When a known quantity of air is passed over red-hot copper turnings, contained in a tube, the whole of the oxygen is seized upon by the copper, and only the nitrogen issues from the tube, and may be collected. Some of the copper is thus converted into oxide, and the increase of the weight of the tube’s contents shows the weight of oxygen contained in the air, while the weight of nitrogen may be known from the volume collected. In this way the chemist analyses atmospheric air, and determines that 100 parts by weight of dry air contain about 79 of nitrogen and 21 of oxygen; or, by measure, about four times as much of the former as of the latter. Now, chemists are acquainted with no fewer than five different substances which contain nothing but nitrogen and oxygen. These substances are either gases, or can be changed into the gaseous form by heat, and they can all be analysed in the same manner as air. The results of such analyses show in 100 parts by weight of each substance the following proportions of its constituents:

┌────────┬──────┬──────┬──────┬──────┬──────┐ │ │No. 1.│No. 2.│No. 3.│No. 4.│No. 5.│ ├────────┼──────┼──────┼──────┼──────┼──────┤ │Nitrogen│63·64 │46·67 │36·84 │30·44 │25·93 │ │Oxygen │36·36 │53·33 │63·16 │69·56 │74·07 │ └────────┴──────┴──────┴──────┴──────┴──────┘

In casting the eye over this table, no relation will probably be detected between the five cases. But if we write down, not the quantities of nitrogen and oxygen contained in 100 parts of each compound, but the quantity of oxygen which in each compound is united to some fixed quantity of nitrogen, we shall at once detect a remarkable law: thus, taking 28 as the fixed weight of nitrogen, for reasons which need not be here explained:

┌────────┬──────┬──────┬──────┬──────┬──────┐ │ │No. 1.│No. 2.│No. 3.│No. 4.│No. 5.│ ├────────┼──────┼──────┼──────┼──────┼──────┤ │Nitrogen│ 28 │ 28 │ 28 │ 28 │ 28 │ │{Oxygen │ 16 │ 32 │ 48 │ 64 │ 80 │ │{ or │16 × 1│16 × 2│16 × 3│16 × 4│16 × 5│ └────────┴──────┴──────┴──────┴──────┴──────┘

Chemists have a sort of shorthand method of expressing the composition of substances, which may be conveniently illustrated by the case before us. Let it be agreed that the letter N shall not only represent nitrogen, but always fourteen parts by weight—grains, ounces, &c., &c.,—of nitrogen; and that, similarly, O shall stand for sixteen parts by weight of oxygen. It is plain that the composition of the compound No. 2 may be represented by simply writing down “NO;” and that of No. 4, in which there is just double the proportion of oxygen, by “NOO.” But to avoid an unnecessary repetition of the same symbol, when it has to be taken more than once, a small figure is written after and a little below it. Thus, for OO, “O_{2}” is written. The proportional composition of each of the five compounds will now be obvious from the following symbols:

┌────────────┬────────────┬────────────┬────────────┬────────────┐ │ No. 1. │ No. 2. │ No. 3. │ No. 4. │ No. 5. │ ├────────────┼────────────┼────────────┼────────────┼────────────┤ │ N_{2}O │ NO │ N_{2}O_{3} │ NO_{2} │ N_{2}O_{5} │ └────────────┴────────────┴────────────┴────────────┴────────────┘

These symbols may be regarded as merely a compendious expression of the composition of each substance—as a shorthand statement of the facts of analysis. But to the majority of chemists the symbols have a deeper significance; for they are taken as representing the atoms of each element which enter into each smallest possible particle of a compound; they express a certain theory of the ultimate constitution of matter. Thus, if we suppose that there exist indivisible particles of nitrogen and of oxygen, and that each smallest particle, or molecule, of the compounds under consideration is constituted of a certain definite and invariable number of each kind of atoms; and, further, if we suppose that an atom of oxygen is heavier than one of nitrogen in the proportion of 16 to 14, or 8 to 7, we shall have a simple theoretical explanation of the relations in the proportions already pointed out. In fact, these would result from the simplest combinations of the two kinds of atoms; and we can picture each one of the smallest particles of the several bodies as thus constituted:

┌────────────┬────────────┬────────────┬────────────┬────────────┐ │ No. 1. │ No. 2. │ No. 3. │ No. 4. │ No. 5. │ ├────────────┼────────────┼────────────┼────────────┼────────────┤ │ ● ● │ │ ● ● │ ● │ ● ● │ │ ○ │ ● ○ │ ○ ○ ○ │ ○ ○ │ ○ ○ ○ │ │ │ │ │ │ ○ ○ │ ├────────────┼────────────┼────────────┼────────────┼────────────┤ │ N_{2}O │ NO │ N_{2}O_{3} │ NO_{2} │ N_{2}O_{5} │ └────────────┴────────────┴────────────┴────────────┴────────────┘

The black circles represent nitrogen atoms, and the open ones oxygen atoms; the symbols are placed below in order that their relation to the supposed atomic constitution may be obvious at a glance. While the symbol of a compound must always accord with its percentage composition, the latter does of itself determine the symbol or formula. A number of other circumstances, which cannot here be discussed, are taken into account as evidence of the constitution of the molecule.

This digression on chemical formulæ will, it is hoped, enable the general reader, who may not previously have been acquainted with them, to perceive their significance, instead of passing them over as unintelligible cabalistic letters when they occur in the following pages. With this object, it may be added that the elements, hydrogen, carbon, and chlorine, are respectively represented by H, C, and Cl; and that the proportional quantities, which are also implied in the symbols, and are those by which H, C, and Cl combine with other bodies, are 1, 12, and 35·5 respectively. Another point which should be understood is that the properties and behaviour of a chemical compound are different, and usually extremely different, from those of any of its constituents. This is well illustrated in the subject we are considering. Atmospheric air is a mixture (not a compound) of nitrogen and oxygen gases, and all its properties are intermediate between those of its ingredients taken separately. Nitrous oxide, N_{2}O, has properties not possessed by either constituent separately. For example, it is very soluble in water, whereas oxygen is very slightly so, and nitrogen still less. The other compounds we have referred to differ widely from nitrous oxide and from each other in their properties.

Nitrous oxide is an invisible gas, having a slightly sweetish taste and smell. It is dissolved by water, which, at ordinary temperatures, takes up about three-fourths of its volume of the gas. By cold and great pressure the gas may be condensed into a colourless liquid. The gas is obtained in a pure state by gently heating the salt called ammonium nitrate, which is formed by neutralizing pure nitric acid with carbonate of ammonia. The action which occurs may be explained thus: the hydrogen of the ammonium unites with a portion of the oxygen of the nitric acid, forming water, whilst the remainder of the oxygen combines with the nitrogen. As chemical actions are regarded as either separations or unions of atoms, they can be expressed by what is called a chemical equation, the left-hand side of which shows the arrangement of the atoms before the action, and the right-hand side the arrangement after it, the sign of equality being read as “produce” or “produces.” But the validity of the equations, like that of the symbolic formulæ, is quite independent of the existence of atoms; for the equation always rests on certain facts, namely, the relations between the quantities of the substances which enter into, and those which are produced by, a chemical action. Thus, in the present case the action may be symbolically expressed as follows:

H_{4}N NO_{3} = 2H_{2}O + N_{2}O Ammonium nitrate. Water. Nitrous oxide.

The equation expresses the fact that every 80 parts by weight of ammonium nitrate, which are used in this reaction, split up into 36 of water and 44 of nitrous oxide.

No attempt seems to have been made to turn Davy’s suggestion to practical account; but in courses of chemical lectures at the hospitals and elsewhere the peculiar physiological properties of nitrous oxide have, since Davy’s announcement, always been demonstrated by some person inhaling the gas. In the medical schools the students often operated on a comrade who was under the influence of nitrous oxide to the extent of bestowing sundry pinches and cuffs, which fully proved the anæsthetic qualities of the nitrous oxide. In 1818 Faraday pointed out the similarity between the effects of ether and of nitrous oxide, and from that time Professor Turner regularly included among the experiments of his course of chemistry the inhalation of the vapour of ether by one of the students. This was done by simply pouring a little ether into a bladder of air, and by means of a tube drawing the mixed air and vapour into the mouth. Until 1844 the effects of nitrous oxide and of ether vapour remained without application, although thus continually demonstrated in lectures. At the close of that year, Mr. Horace Wells, a dentist, of Hartford, Connecticut, U.S.A., witnessed the usual experiments with nitrous oxide at a public lecture. At his request the lecturer attended at Mr. Wells’s residence the following day, to administer to him the nitrous oxide, in order that he might try its efficacy in annulling pain, for he was himself to have a tooth extracted by a brother dentist. His exclamation on finding the operation painlessly over was, “A new era in tooth-pulling!” Mr. Wells continued his experiments on the use of nitrous oxide in dental operations, but he did not apparently obtain uniform results, for he pronounced its effects uncertain, and he gave it up. On the occasion when Mr. Wells’s tooth was extracted, Dr. W. T. G. Morton was present, and he soon afterwards found that under the influence of ether vapour, teeth might be painlessly extracted and surgical operations performed. Dr. Morton attempted to conceal the substance he used under the name of “letheon,” for which he obtained a patent. But the well-known and characteristic odour of ether declared the nature of the “letheon;” and Dr. Bigelow having in consequence tried ether, found it to produce all the effects of “letheon.” So the matter was no longer a secret. Dr. Morton was, therefore, the person who first applied ether vapour, and the extraction of a tooth was the occasion of its first application. This was in 1846. It was used for the first time in England on the 19th of December, 1846, also for the extraction of a tooth; and two days afterwards Mr. Liston, the eminent surgeon, performed the operation of amputating the thigh while his patient was under the influence of ether. The employment of ether in surgical operations quickly spread, and its administration in hospitals became general throughout Europe and America.

The chemical constitution of ether, and its relation to alcohol, may be indicated by the following formulæ:

HOH HO(C_{2}H_{5}) (C_{2}H_{5})O(C_{2}H_{5}) Water. Alcohol. Ether.

If we suppose one of the hydrogen atoms in the molecule of water to be removed and replaced by the group (C_{2}H_{5}), the result is alcohol. If, now, (C_{2}H_{5}) be substituted in the alcohol for the remaining atom of hydrogen, we get a particle of ether. Ether was discovered in 1540, and described as sweet oil of vitriol, but its real nature was first pointed out by Liebig. It is prepared by distilling a mixture of sulphuric acid and alcohol. It is a colourless transparent liquid, extremely volatile, and possessing a peculiar and powerful odour. It evaporates so rapidly that a drop allowed to fall from a bottle on a warm day may be converted into vapour before it reaches the ground. When its vapour is inhaled in sufficient proportion mixed with air, it soon produces a complete insensibility to pain. In the case of a full-grown man who inhales air containing 45 per cent. of the vapour, about 2 drams per minute of the liquid are consumed. The air is allowed to stream over the surface of the liquid in a proper apparatus, where it takes up the vapour, and the two pass through a flexible tube to a piece fitting over the mouth and nostrils of the patient. The effects produced are progressive, and may be thus described:

For about two minutes after the beginning of the inhalation, the patient retains his mental faculties, and has some power of controlling his movements, but in a confused and disordered manner. At the end of the third minute he is unconscious; there are no voluntary movements, but muscular contractions may agitate the frame. At the end of the fourth minute, the only perceptible movements are the motions of the chest in respiration. If the inhalation be discontinued at the end of the fourth minute, when 1 oz. of ether will have evaporated, similar stages are passed through in reverse order during recovery. The condition reached at the end of the fourth minute continues about two minutes; the intermediate state lasts three or four minutes; the condition of confused intellect and will about five minutes. This is succeeded by a feeling of intoxication and exhilaration, which continues for ten or fifteen minutes. It was probably this excitement of the system produced by ether which has caused it to be superseded—in Britain, at least—in about twelve months after its adoption, by chloroform.

Chloroform appears to have been independently discovered in 1831, by Soubeiran, and by an American chemist, Guthrie. It is usually procured by distilling a mixture of bleaching powder, spirits of wine, and water. Chloroform is a colourless volatile liquid, of an odour much more agreeable than that of ether. Its composition is represented by CHCl_{3}. The merit of having first applied the singular properties of this substance to the alleviation of human suffering belongs to the late Sir J. Y. Simpson, of Edinburgh. Its use as an anæsthetic was apparently suggested to this eminent professor by Mr. Waldie, of Liverpool. It was first applied at Edinburgh on the 15th November, 1847; and when its efficacy had been proved, it was soon extensively used, and in Europe, at least, almost entirely superseded ether, as being more rapid and certain in its action, not producing injurious excitement, and being pleasanter to inhale. A notion prevailed that chloroform was not only more powerful in its operation than ether, but also more safe. In January, 1848, its administration, however, proved fatal to a patient; and since then a certain number of casualties of this kind have occurred with chloroform, ether, and other anæsthetics.

The patient is often made to inhale the vapour of chloroform by merely holding before his mouth and nostrils a sponge or handkerchief, on which a small quantity of the liquid has been poured. Dr. Snow contrived an apparatus for administering the vapour with more regularity. A metal box adapted to the shape of the face is made to cover the mouth and nostrils. This piece has two valves, one of which admits the air and vapour from an elastic tube connected with the apparatus containing the chloroform, and prevents its return; the other valve is a flap opening outwards, which allows the expired air to escape. There is also an adjustment for admitting directly into the mouthpiece more or less atmospheric air.

The sensations first experienced when chloroform is inhaled are said to be agreeable. Many persons have described the feeling as resembling rapid travelling in a railway carriage; there is a singing in the ears, and when the power of vision ceases, and the person is no longer conscious of light, the sensation is that of entering a tunnel. After this there is a lessened sensibility to pain; and in the next stage the unconsciousness to outward impressions is deeper, but the mental faculties, though impaired, are not wholly suspended, for the patient may speak, and usually dreams something which he afterwards remembers. When the person is still more under the influence of the chloroform, no voluntary motions take place, although there may be some inarticulate muttering. Dr. Snow describes several conditions which may be observed in patients undergoing operations under the influence of chloroform. First, the patient may preserve the most perfect quietude without a sign of consciousness or sensation; this is the most usual condition. Second, he may moan, or cry, or flinch under the operation, without, however, having the least memory of any pain when he recovers. Third, the patient may talk, laugh, or sing during the operation; but what he says is altogether devoid of reference to what is done. Fourth, he may be conscious of what is taking place, and may look on while some minor operation is proceeding, without feeling it, or without feeling it painfully. This is often the condition of the patient as the effect is passing off, while some smaller operation is still proceeding. Fifth, the patient may complain he is being hurt; but afterwards, when the effect of the chloroform has passed off, he will assert that he felt no pain whatever. When the chloroform has been inhaled for but a short time, the patient becomes conscious in about five minutes after its discontinuance; but with a longer inhalation the period of unconsciousness may last for perhaps ten minutes. The return of consciousness takes place with tranquillity: not unfrequently the patient’s first speech, even after a serious operation, often being an assertion that the chloroform has not taken effect.

In the strongest degree of ether and chloroform effects, all the muscles of the body are relaxed; the limbs hang down, or rest in any position in which they are placed; the eyelids droop over the eyes, or remain as they are placed by the finger; the breathing is deep, regular, and automatic; there is often snoring, and this is, indeed, characteristic of the deepest degree of unconsciousness; the relaxation of the muscles renders the face devoid of expression, and with a placid appearance, as if the person were in a sound natural sleep. He is perfectly passive under every kind of operation. The breathing and the action of the heart proceed all the while with unimpaired regularity. It is, however, known by experiments on animals that if the inhalation be prolonged beyond the period necessary to produce these effects, the respiratory functions are interfered with by the insensibility extending to the nerves on which they depend. The breathing of an animal thus treated becomes irregular, feeble, or laborious, and death ensues. However nearly dead from inhalation of ether vapour the animal may be, provided respiration has not actually ceased, it always recovers when allowed to breathe fresh air. Of course, the etherization is never carried to this stage with human beings.

Air containing 2 grs. of chloroform in 100 cubic inches suffices to induce insensibility; but 5 grs. in 100 cubic inches is found a more suitable proportion. Dr. Snow, who strongly disapproved of the uncertain and irregular mode of administering chloroform on a handkerchief or sponge, contrived the inhaling apparatus already described. The air before reaching the mouth and nostrils of the patient passes through a vessel containing bibulous paper moistened with chloroform. This vessel he surrounds with water at the ordinary temperature of the air, in order to supply the heat absorbed by the conversion of the liquid into vapour, so that the formation of the latter may go on regularly. The same thoughtful arrangement formed part of the ether-inhaler he had previously contrived.

The extraordinary effects of ether and chloroform have introduced new and important facts into psychological science, and have illustrated and extended some of the most interesting results of physiological research. Let us trace the action of these substances, and explain it as far as may be. Nitrous oxide, ether vapour, and chloroform vapour are all soluble in watery fluids. The lungs present a vast surface bathed by watery fluids, and therefore these gases are largely absorbed; and by a well-known process, they pass directly into the blood, through the delicate walls of the capillary vessels. The odour of ether can be detected in any blood drawn from persons under its influence. Ether, or chloroform, thus brought into the general current of the circulation, is quickly carried to all parts of the body, and thus reaches the nerve-centres. On these it produces characteristic effects by suspending or paralysing nervous action: why or how this effect takes place is unknown. The nervous centres are not all acted upon in an equal degree—some require a larger quantity of the drug to affect them at all. The parts of the nervous system first affected are the cerebral lobes, which are known to be the seat of the intellectual powers. The cerebellum—the function of which there is reason to believe is the regulation and coordination of movements—is the next to yield to the influence. Then follow the spinal nerves, which are the seat of sensibility and motive power. This is as far as the action can safely be carried: the nervous centre called the medulla oblongata, which is placed at the junction of the brain and the spinal cord, still performs its functions—one of the most important of which is to produce the muscular contractions that keep the respiratory organs in action. We have seen, by the effects of further etherization in animals, that when this part of the system is affected, the animal dies from a stoppage of the respiration.

But, unfortunately, there have been instances in which death has been caused by the administration of ether and chloroform even under the most skilful management. But these occurrences were not the result of the inhalation having been carried so far as to stop respiration: in some cases the patient has died before the first stage of insensibility. These fatal cases have all been marked by a sudden paralysis of the heart—that organ has abruptly ceased to act. Why in these, certainly a very small percentage of patients, the action of the drug should at once take effect on the heart has not yet been explained. The rhythmic action of the heart depends upon nervous centres enclosed within its own substance, so that this organ is to a certain extent independent; but it is connected with the other nervous centres by the branches of a remarkable nerve which proceeds from the medulla oblongata, and also by another set of nerves which come from the chain of ganglia called the sympathetic nerve. The nerve connecting the heart with the medulla is a branch of that called the pneumo-gastric, and it is a well-established fact that the action of the heart may be arrested by irritation of this nerve. The comparatively few fatalities which have attended the use of anæsthetics may, therefore, be due either to an immediate action on the nerve-centres of the heart, or possibly to a mediate action through the medulla and the pneumo-gastric nerve.

Soon after the introduction of ether the use of nitrous oxide was discontinued by the dentists, on account of the apparent uncertainty of its action. Within the last few years, however, its employment in the extraction of teeth has been revived by Dr. Evans, of Paris, who found that to insure certainty in its action, the great point is the inhalation of the gas in a pure state and without admixture of air. Nitrous oxide seems now to be extensively used by dentists, and thus Davy’s experiment of 1800 is repeated and verified daily in thousands of cases, and to the great relief of hundreds who probably never heard his name.

Other bodies, such as amylene (C_{5}H_{10}), carbon tetrachloride (CCl_{4}), &c., have been tried as substitutes for ether and chloroform; but having been found less efficacious or more dangerous, their use has been abandoned. It might be instructive to reflect how much unnecessary pain would have been spared to mankind had ether and chloroform been known and applied at an earlier age. We know not what other beneficent gifts chemistry may yet have in store for the alleviation of suffering, but it is unlikely that even ether and chloroform are her derniers mots. It should be remembered that the chemists who discovered and examined these bodies were attracted to the work by nothing but the love of their science. They had no idea how invaluable these substances would afterwards prove. The chemist of the present day, whose labour is often its own reward, may be cheered and stimulated in his toil by the thought that while no discovery is ever lost, but goes to fill its appropriate place in the great edifice of science, even the most apparently insignificant truth may directly lead to invaluable results for humanity at large.

What strange things the ancient thaumaturgists might have done had they been possessed of the secret of chloroform or of nitrous oxide! What miracles they would have wrought—what dogmas they would have sanctioned by its aid! But the remarkable effects produced by the inhalation of certain gases or vapours were not altogether unknown to the ancients—although these effects were then attributed to anything but their real cause. It is related that a number of goats feeding on Mount Parnassus came near a place where there was a deep fissure in the earth, and thereupon began to caper and frisk about in the most extraordinary manner. The goatherd observing this, was tempted to look down into the hole, to see what could have caused so extraordinary an effect. He was himself immediately seized with a fit of delirium, and uttered wild and extravagant words, which were supposed to be prophecies. The knowledge of the presumed divine inspiration spread abroad, and at length a temple in honour of Apollo was erected on the spot. Such was the origin of the famous Oracle of Delphi, where the Pythoness, the priestess of Apollo, seated on a tripod placed over the mysterious opening, delivered the response of the god to such as came to consult the oracle. It is stated by the ancient writers, that when she had inhaled the vapour, her eyes sparkled, convulsive shudders ran through her frame, and then she uttered with loud cries the words of the oracle, while the priests who attended took down her incoherent expressions, and set them in order. These possessions by the spirit of divination were sometimes violent. Plutarch mentions a priestess whose frenzy was so furious, that the priests and the inquirers alike fled terrified from the temple; and the fit was so protracted that the unfortunate priestess herself died a few days afterwards.

EXPLOSIVES.

The illustration above will serve to remind the reader of the great importance of explosive agents in the operations of civil industry. By reason of the more impressive and exciting spectacles which attend the use of such agents in warfare, we are rather apt to lose sight of their far more extensive utility as the giant forces whose aid man invokes when he wishes to rend the rock in order to make a road for his steam horse, or in order to penetrate into the bowels of the earth in search of the precious ore. A little reflection will show that if such work had to be done with only the pickaxe, the chisel, and the crowbar, the progress would be painfully slow; and railway cuttings through masses of compact limestone, like that represented in Fig. 339, for example, would be well-nigh impossible. The formation of cuttings and tunnels, and the removal of rocks in mining operations, are not the only service which explosive agents render to the industrial arts; there is, besides other uses which might be enumerated, the preparation of foundations for buildings, bridges, harbours, and lighthouses. The use of gunpowder in all such operations as those which have been referred to is too well known to require description. But of late years gunpowder has been to a great extent superseded for such purposes by two remarkable products of modern chemistry, called gun-cotton and nitro-glycerine. Military art has also benefited by at least one of these products; and the use of charges of gun-cotton for torpedoes has already been described and illustrated in these pages.

It is not a little curious that the two most terribly powerful explosives known to science should be prepared from two most harmless and familiar substances. The nice, soft, clean, gentle cotton-wool, in which ladies wrap their most delicate trinkets, becomes, by a simple chemical transformation, a tremendously powerful explosive; and the clear, sweet, bland liquid, glycerine, which they value as a cosmetic for its emollient properties, becomes, by a like transformation, a still more terrifically powerful explosive than the former. It is, perhaps, even more curious that having undergone the transformation which confers upon it these formidable qualities, neither cotton-wool nor glycerine is changed in appearance. The former remains white and fleecy; the latter is still a colourless syrupy-looking liquid.

The fibres which form cotton, linen, paper, and wood, are composed almost entirely of a substance which is known to the chemist as cellulose or cellulin. That this substance, as it exists in the fibres of linen and in sawdust, could be converted into an explosive body by the action of nitric acid, appears to have been first observed by the French chemist, Pelouze, in 1838. The action with cellulose in the form of cotton-wool was more fully examined by Professor Schönbein, of Basle, who, in 1846, first described the method of preparing gun-cotton, and suggested some uses for it. He directs that one part of finely-carded cotton-wool should be immersed in fifteen parts of a mixture of equal measures of strong sulphuric and nitric acids; that after the cotton has remained in the mixture for a few minutes, it should be removed, plunged in cold water, and washed until every trace of acid has been removed, and then carefully dried at a temperature not exceeding the boiling-point of water.

After Professor Schönbein had demonstrated the power of the new agent in blasting, and its projectile force in fire-arms, its manufacture on a large scale was undertaken at several places. Messrs. Hall commenced to make it at their gunpowder works at Faversham, and a manufactory was also established near Paris. In July, 1847, a fearful explosion of gun-cotton occurred at the Faversham works, which was believed to have been caused by the spontaneous detonation of that substance. This induced Messrs. Hall to discontinue the manufacture as too dangerous; and they even destroyed a large quantity of the product which they had in hand by burying it in the ground. The making of gun-cotton was soon afterwards discontinued also by the French, who did not find the substance to possess all the qualities fitting it for military use. The Prussian Government also began to make gun-cotton; but the experiments were put a stop to by the explosion of their factory. An eminent artillery officer in the Austrian service, General von Lenk, undertook a thorough examination of the manufacture and properties of gun-cotton for military purposes. He introduced several improvements into the processes of the manufacture; and the Austrian Government established works at Hïrtenberg, with a view to the adoption of gun-cotton as a substitute for gunpowder in fire-arms. It has some undoubted advantages over powder, for it neither heats the gun nor fouls it, and it produces no smoke. Notwithstanding this the Austrians have not abandoned the use of gunpowder in favour of gun-cotton.

Gun-cotton, as a military agent, has a strenuous advocate in Professor Abel, who presides over the Chemical Department of the British War Office. To this gentleman we are indebted for great improvements in the manufacture of gun-cotton, and for a more complete investigation of its properties. Professor Abel’s processes were put in practice at a manufactory which the Government established at Waltham Abbey; and Messrs. Prentice also set up works at Stowmarket.

Some details of the mode in which the manufacture of gun-cotton was carried on at Stowmarket may be of interest. The cotton was first thoroughly cleansed and carefully dried; and these operations are of great importance, for unless they are well performed, the product is liable to explode spontaneously. The cotton was then weighed out in charges of 1 lb., and each charge was completely immersed in a separate vessel, containing a cold mixture of sulphuric and nitric acids. After a short immersion the cotton was removed from the liquid, and with about ten times its own weight of acids adhering to it, each charge was placed in a separate jar, where it was allowed to remain for forty-eight hours. The vessels were kept cool during the whole period by being placed in a trough through which cold water was flowing. On removal from the jars, the cotton was freed from adhering acid by being placed in a centrifugal drying machine. It was then drenched with a large quantity of cold water, and dried, washed again in a stream of cold water for forty-eight hours, and the operations of alternately washing for forty-eight hours and drying were repeated eight times. The drying was effected by placing the material in cylinders of wire-gauze, which were whirled round by a steam engine at the rate of 800 revolutions per minute, so that the water was expelled by centrifugal force. The cotton was next reduced to a pulp by a process similar to that which is employed in paper-making, and the moist pulp was rammed into metallic cylinders by hydraulic pressure, in order that it might be brought into forms suitable for use in blasting, &c. The pulp was put into these moulds while wet, but the water was nearly all expelled by the compression. The cylinders of gun-cotton thus obtained were then covered with paper-parchment, and finally dried at a steam temperature, with many precautions. The compression of the cotton pulp, by bringing a large quantity of the material into a smaller bulk, causes a greater concentration of the explosive energy, and this is a matter of great importance in blasting.

We may now consider what chemistry has to teach concerning the nature of the action by which cotton-wool is converted into gun-cotton. Cotton itself is nearly pure cellulose. The chemical composition of cellulose may be represented most simply by the formula C_{6}H_{10}O_{5}. Nitric acid is a powerful oxidizing agent, and is constantly used in chemistry to fix oxygen in various substances; but another kind of action exerted by nitric acid in certain cases consists in the substitution of a portion of its atoms for hydrogen, by which the residue of the particle of nitric acid is converted into water. The formula for nitric acid may be written HO NO_{2}, and it will be seen that by changing NO_{2} for H, water, HOH, would be produced. This is precisely the kind of action which occurs when cellulose is converted into nitro-cellulose. Two or three, or more, atoms of hydrogen may be taken out of cellulose, and replaced by two or three, or more, groups NO_{2}, and the result will be a different kind of nitro-cellulose, according to the number of atoms in the molecule replaced by NO_{2}. Several varieties of gun-cotton are known, these being doubtless the result of the differences here alluded to. The action producing di-nitro-cellulose is represented by this equation:

C_{6}H_{10}O_{5} + 2HNO_{3} = C_{6}H_{8}(NO_{2}){2}O{5} + 2H_{2}O. Cellulose. Nitric acid. Di-nitro-cellulose. Water.

The equation shows that water is produced by the reaction, and the sulphuric acid which is used in the preparation performs no further part than to take up this water, which would otherwise go to dilute the rest of the nitric acid. The union of sulphuric acid and water is attended with great heat, hence the necessity of cooling the vessels in making the gun-cotton. Quite other products would be formed if the mixture became heated.

The action of nitric acid on glycerine is of the same kind as that on cellulose. When glycerine is allowed to drop into a cooled mixture of nitric acid and sulphuric acid, the eye can detect little or no difference between the appearance of the liquid which collects in the bottom of the vessel and the glycerine dropped in. The product of the action is, however, the terrible nitro-glycerine, a heavy, oily-looking liquid, which explodes with fearful violence. Even a single drop placed on a piece of paper, and struck on an anvil, detonates violently and with a deafening report. The chemical change which is effected in the glycerine (C_{3}H_{8}O_{3}), is the substitution of three NO_{2} groups for three of hydrogen, producing C_{3}H_{5}(NO_{2}){3}O{3}, or tri-nitro-glycerine. The general reader may perhaps marvel that the chemist should be able not only to count the number of atoms which go to make up the particles of a compound body, but to say that they are arranged so and so: that the atoms do not form an indiscriminate heap, but that they are connected in an assignable manner. The reader is no doubt aware that these compound particles are extremely small, and he may reasonably wonder how science can pronounce upon the structure of things so small. He may be more perplexed to learn that a calculation made by Sir W. Thompson shows that the particles of water, for instance, cannot possibly be more than the 1/250000000th of an inch in diameter, and may be only 1/20th of that size. The truth is that the very existence of atoms and molecules is an assumption. Like the undulatory ether, it is an hypothesis which is adopted to simplify and connect our ideas, and not a demonstrated reality. But the atomic hypothesis has so wide a scope that some philosophers hold the existence of atoms and molecules as almost a known fact. Be that as it may, the chemist in assigning to a body a certain molecular formula really does nothing but express the results of certain experiments he has made upon it. With one re-agent it is decomposed in this manner, with another in that. By certain treatment it yields an acid, a salt; so much carbonic acid, such a weight of water, is acted on or remains unaltered; gives a precipitate or refuses to do so. Such are the facts which the chemist conceives are co-ordinated and expressed by the formula he gives to a substance. The best formula is that which accords with the greatest number of the properties of the body—which includes as many of the facts as possible. It follows, therefore, that a formula which aims at expressing more than the mere percentage composition of the body—which, in the language of the atom hypothesis, seeks to represent the mode in which the atoms are grouped in the molecule, but which in reality represents only reactions, is written according as the chemist considers this or that group of reactions more important. These remarks might be illustrated by filling this page with the different formulæ (a score or more) which have been proposed as representing the constitution (reactions?) of one of the best-known of organic compounds, namely, acetic acid.

Whether atoms really exist, and their arrangement in the particles of bodies can be deduced from the phenomena, or not, the fact is undeniable that these ideas have given chemists a wonderful grasp of the facts of their science. The consistency and completeness of the explanation afforded by these theories are ever being extended by modifications which enable them to embrace more and more facts. Some of the properties of the substance we are now considering confirm in a remarkable manner the theoretical views which are expressed in its constitutional formula. We may first consider the nature of gunpowder, and by comparing it with nitro-glycerine, endeavour to explain the greater power of the latter substance. Gunpowder is a mixture of charcoal, sulphur, and nitre, the latter constituting three-fourths of its weight. Nitre supplies oxygen for the combustion of the charcoal, which is thus converted into carbonic acid, and the sulphur, which is added to increase the rapidity of the combustion, is also oxidized. The products of the action are, however, numerous and complicated, but the important result is the sudden generation of a quantity of carbonic acid, nitrogen, carbonic oxide, hydrogen, and other gases, which at the oxidizing temperature and pressure of the air would occupy a space 300 times greater than the powder from which they are set free; but the intense heat attending the chemical action dilates the gases, so that at the moment of explosion they would occupy a space at least 1,500 times greater than the gunpowder. The materials of which gunpowder is composed are finely powdered, in order that each portion shall be in immediate contact with others, which shall act upon it. Plainly, the more thorough the incorporation of the materials—that is, the more finely ground and intimately mixed they are—the more rapid will be the inflammation of the powder.

Looking now at the crude formula of nitro-glycerine, C_{3}H_{5}N_{3}O_{9}, the reader will remark that the molecule contains more than sufficient oxygen to form carbonic acid with all the carbon atoms, and water with all the hydrogen atoms; for the C_{6} in two molecules of nitro-glycerine would take only O_{12} to form 6CO_{2}; and the H_{10}, to be converted into 5H_{2}O, would only need O_{5}; thus there would be an excess of oxygen. Now, it may occur to the reflective reader that in every molecule of nitro-glycerine the carbon and hydrogen are already associated with as much oxygen as they can take up: that they are, in fact, already burnt, and that no further union is possible. But from chemical considerations it has been deduced that in the nitro-glycerine molecule the oxygen atoms, except only three, which are partially and imperfectly joined to carbon, are united to nitrogen atoms only. The constitution of the molecule may be represented by arranging, as below, the letters which stand for the atoms, and by joining them with lines, which shall stand for the bonds by which the atoms are united.

O H H H O | | | | | N—O—C—C—C—O—N | | | | | O H O H O | O=N=O

We see here that the hydrogen atoms are completely, and the carbon atoms partially, detached from the oxygen atoms; and therefore these atoms are in the condition of the separated carbon and oxygen atoms in gunpowder. Only the pieces of matter which lie side by side in gunpowder are in size to the molecules of nitro-glycerine as mountains to grains of sand. The mixture of the materials is then so much more intimate in nitro-glycerine, since atoms which can rush together are actually within the limits of the molecules; and these molecules have such a degree of minuteness, that 25 millions, at least, could be placed in a row within the length of an inch. We know that the finer the grains and the more intimate the mixture, the quicker will gunpowder inflame; but here we have a mixture far surpassing in minute subdivision anything we can imagine as existing in gunpowder. Hence the combination in the case of nitro-glycerine must be instantaneous, whereas that in gunpowder, quick though it be, must still require a certain interval. If it take a thousandth of a second for the gases to be completely liberated from a mass of gunpowder, and only one-millionth of a second for a vast quantity of carbonic acid, nitrogen, and steam to be set free from nitro-glycerine, the destructive effect will be much greater in the latter case. Again, the volume of the gases liberated from nitro-glycerine in its detonation have at least 5,000 times the bulk of the substance. We have entered into these chemical considerations, at some risk of wearying the reader, with the desire of affording him a clue to the singular properties of nitro-glycerine and gun-cotton, which we are about to describe.

The nature of the chemical changes which may be set up in an explosive substance, and the rapidity with which these changes proceed throughout a mass of the material, are greatly modified by the conditions under which the action takes place. If a red-hot wire be applied to a small loose tuft of gun-cotton, it goes off with a bright flash without leaving any smoke or any other residue. Thus, when the substance is quite unconfined, no explosion occurs. If the cotton-wool be made into a thread, and laid along the ground, it will burn at the rate of about 6 in. per second; if it be twisted into a yarn, the combustion will run along at the rate of 6 ft. per second; but if the yarn be enclosed in an Indian-rubber tube, the ignition proceeds at the rate of 30 ft. in a second. If to a limited surface of gun-cotton, such as one end of a length of gun-cotton yarn, a source of heat is applied—the temperature of which is high enough to set up a chemical change, but not high enough to inflame the resulting gases (carbonic oxide, hydrogen, &c.)—the cotton burns comparatively slowly, rather smouldering than inflaming. If, however, a flame be applied, the gun-cotton flashes off with great rapidity, because the heat applied sets fire to the gaseous products of the chemical action. But if the gun-cotton be confined so that the gases cannot escape, the combustion becomes rapid however set up. The reason is that if the gases escape into the air, they carry off so much of the heat produced by the smouldering gun-cotton, that the temperature does not rise to the extent required to produce the flaming ignition, in which the products are completely oxidized. If a mass of gun-cotton be enclosed in a capacious vessel from which the air has been removed, and the gun-cotton be ignited by means of a wire made hot by electricity, the cotton will at first only burn in the slow way without flame; but as the gases accumulate and exert a pressure which retards the abstraction of heat accompanying their formation, the temperature will rise and attain the degree necessary for the complete and rapid chemical changes involved in the flaming combustion. Thus, the more resistance is offered to the escape of the gases, the more rapid and perfect is the combustion and explosive force produced by the ignition. Now, the explosion of gun-cotton has been found to be too rapid when it is packed into the powder-chamber of a gun, for its tendency is to burst the gun before the ball has been fairly started. Hence a material like gunpowder, in which the combustion is more gradual, is better suited for artillery. The ignition of gunpowder, though rapid, is not instantaneous, and therefore we can speak of it as more or less gradual. Indeed, in even the most violent explosives, some time is doubtless required for the propagation of the action from particle to particle. This extreme rapidity of combustion, and consequent rending power, which is so objectionable in a gun-chamber, makes gun-cotton a most powerful bursting charge for shells, and, when it is enclosed in strong receptacles, for torpedoes also.

But by the researches of Nobel, Professor Abel, and others, it has been discovered—and this is, perhaps, the most remarkable discovery in connection with explosives—that gun-cotton, nitro-glycerine, and other explosive bodies, are capable of producing explosions in a manner quite different from that which attends their ignition by heat. The violence of this kind of explosion is far greater than that due to ordinary ignition, for the action takes place with far greater rapidity throughout the mass, and is, indeed, practically instantaneous. It appears to be produced by the mere mechanical agitation or vibrations which are communicated to the particles of the substance. Turning back to the representation of the molecule of nitro-glycerine on page 744, it will not be difficult to imagine that this may be an unstable kind of structure; that the atoms of oxygen are prevented from rushing into union with those of hydrogen and carbon only by the interposition of the nitrogen; and that an agitation of the structure might shake all the atoms loose, and leave them free to re-combine according to their strongest affinities. Nitro-glycerine is by no means so ready to inflame as is gun-cotton: it is said that the flame of a match may be safely extinguished by plunging it into the liquid; and when a sufficient heat is applied to a quantity of the liquid in the open air, it will burn quietly and without explosion. Even when nitro-glycerine is confined, the application of heat cannot always be made to produce its explosion; or, at least, the circumstances under which it can do so are not accurately known, and the operation is difficult and uncertain. On the other hand, nitro-glycerine explodes violently even when freely exposed to the air if there be exploded in contact with it a confined charge of gunpowder, or a detonating compound such as fulminating powder. Gun-cotton possesses the same property of exploding by concussion, which appears indeed to be a general one belonging to all explosive bodies. According to recent researches, even gunpowder is capable of a detonative explosion. A mass of gunpowder confined with a certain proportion of gun-cotton, which is itself set off by fulminate of mercury, is said to exert an explosive force four times greater than that developed by the ignition of the gunpowder in the ordinary manner. It has also been found that wet gun-cotton can be exploded by concussion, and the force of the explosion is unimpaired even when the material is saturated with water. This makes it possible to use gun-cotton with greater safety, as it may be transported and handled in the wet condition without risk, and it preserves its properties for an indefinite period without being deteriorated by the water. Some experiments illustrating the extraordinary force of the detonative explosions of gun-cotton and nitro-glycerine will give the reader an idea of their power.

A palisade, constructed by sinking 4 ft. into the ground trunks of trees 18 in. in diameter, was completely destroyed in some experiments at Stowmarket by the explosion of only 15 lbs. of gun-cotton. Huge logs were sent bounding across the field to great distances, and some of the trees were literally reduced to match-wood. The gun-cotton, be it observed, was simply laid on the ground exposed to the air. The destructive powers of nitro-glycerine are even greater. A tin canister, containing only a few ounces of nitro-glycerine, is placed, without being in any way confined, on the top of a smooth boulder stone of several tons weight; it is exploded by a fuse containing fulminating powder, which is fired from a distance by electricity. There is a report, and the stone is found in a thousand fragments. The last experiment shows one of the advantages of nitro-glycerine over gunpowder as a blasting material, beyond its far greater power, which is about ten times that of gunpowder. A charge of gunpowder inserted in a vertical hole tends to force out a conical mass, the apex of which is at the space occupied by the charge. With nitro-glycerine, and also with gun-cotton, which last has almost six times the force of gunpowder, a powerful rending action is exerted below as well as above the charge. Again, in blasting with gunpowder the charge must be confined, and the hole is filled in above the charge with tightly rammed materials, forming what is termed the tamping. But nitro-glycerine requires no tamping: a small, thin metallic core containing the charge is simply placed in the drill-hole, or the liquid itself is poured in, and a little water placed above it. The effect of the explosion of nitro-glycerine in “striking down,” when apparently no resistance is offered, will seem very strange to the reader who is oblivious of certain fundamental principles of mechanics. The force of the explosion is due entirely to the sudden production of an enormous volume of gas, which at the ordinary pressure would occupy several thousand times the bulk of the material from which it is produced. This gas, by the law of the equality of action and reaction, presses down upon the stone with the same force that it exerts to raise the superincumbent atmosphere. The pressure of the gas at the moment of its liberation is enormous; but the atmosphere cannot instantaneously yield to this, for time is required to set the mass of air in motion, and the wave of compression advances slowly compared with the rapidity of the explosion. Hence the air acts, practically, like a mass of solid matter, against which the gases press, and which yields less readily than the rock, so that the blow which is struck takes visible effect on the latter. Now, with gunpowder, the evolution of gas is less rapid, the atmosphere has time to yield, and the reaction has not the same violence. The rapidity of the evolution of gas from the exploding nitro-glycerine is so great, that the gases, though apparently unconfined, are not so in reality; for the atmosphere acts as a real and very efficient tamping.

When nitro-glycerine first came into use for blasting purposes, it was used in the liquid form under the name of “blasting oil;” but the dangers attending the handling of the substance in this state are so great, that it is now usual to mix the liquid with some powdered substance which is itself without action, and merely serves as a vehicle for containing the nitro-glycerine. To mixtures of this kind the names “dynamite,” “dualine,” “lithofracteur” &c., have been given.

It is now hardly necessary to point out that the discovery of these new explosives has largely extended our power over the rocks, enabling works to be executed which would have been considered impracticable with less powerful agents. It is true that the most fearful disasters have been accidentally produced by the new explosives; but such occasional devastation is the price exacted for the possession of powerful agents. And just as in other cases—steam, for example—where great forces are dealt with, so these new powers must be managed with unceasing care, and placed in the hands of only skilful and intelligent men.

The products of the combustion of gunpowder are not all gaseous, but include solid compounds, such as carbonate and sulphate of potassium. It is these that give rise to the smoke seen when a gun is discharged, and which, in rapid firing, soon obscures the sight of the objects aimed at. They are also the causes of the fouling of the bore. Gun-cotton is quite unexceptionable in these respects, and that prompted the attempts made soon after its introduction to use it instead of gunpowder in fire-arms. But the explosion of gun-cotton was found too sudden and violent for guns and rifles, so that many serious accidents in consequence occurred. The next thing done was to lessen the rapidity of the explosion by using gun-cotton mixed with ordinary cotton, or twisted in threads round some inert substance—in fact, to mitigate the violence of the shock by some mechanical disposition of the material. The introduction of rapid firing guns and repeating rifles forced on the problem of a smokeless powder; and as the plan of replacing nitrate of potassium, in ordinary gunpowder, by nitrate of ammonium was found to be attended with loss of the keeping quality of the powder, other materials, such as picric acid, which forms also the basis of the explosive called mélinite, have been proposed. The composition of mélinite was long a mystery, and that of the smokeless powder adopted by the French was so carefully concealed that many experiments had to be made by other nations to discover some similar preparation, which was found possible by combining certain substances with gun-cotton so as to modify the violence of its explosion, and produce a manageable material having the required properties. The British Government, after many experiments and much careful testing, decided to adopt cordite, made of nitro-glycerine, in which, by the aid of volatile solvent, di-nitro-cellulose is dissolved, together with a little mineral oil. The semi-fluid composition, forced through a round hole or die, comes out like a thread or cord, which the evaporation of the volatile solvent leaves with very much the appearance of common brown window-cord. This material has the several advantages of keeping well, of being uniform in its propulsive powers, of being capable of imparting as high a velocity as a much larger charge of the ordinary black gunpowder, while at the same time exercising a less pressure on the chase of the gun.

It will have become obvious from the preceding paragraphs that, according to the conditions under which an explosive is to be used, selection must be made of the most suitable. For example, the substances employed for propelling projectiles from guns must not have the violent rending power of certain others, which, by this very property, are most useful for blasting operations; and, again, although explosives of this last kind are inadmissible as projectile agents, they are of the kind best adapted for use in shells where it is the disruptive action that is required. Also in blasting operations, the explosive has to be adapted to the nature of the work, and it has been found that a substance which has worked well in driving a heading for a tunnel through one kind of rock may prove both slow in progress, and more costly in expenditure, when some different kind of rock is reached. Besides this, regard must be had in blasting operations to the nature of the effect required, which is in some instances a shattering of the rock into fragments, in others a detachment of it in masses. Thus, in the working of a slate quarry, the explosive used must not be of a nature to shiver the rock into useless splinters, but must operate in such a manner that compact masses may be separated from the mountain side in a condition suitable for cleaving, by appropriate tools, into numberless broad laminæ, which, trimmed into rectangular shape, constitute our well-known roofing slates. The blasting used on a coal seam must be so conducted as to yield the material as much as possible in big lumps or cobbles rather than in slack. When granite is blasted for the purpose of obtaining building stones, the explosive must be one that, by its comparatively slow action, divides the compact rock into the largest possible blocks. On the other hand, when granite is blasted merely with the object of removing it, as when a tunnel has to be driven through a mass of it, the most disintegrating agent is then the best. The common popular expressions by which the two classes of explosives just referred to are distinguished are “high explosives” and “low explosives.” Dynamite may be taken as a type of the former, and gunpowder a type of the latter. As will be gathered from what is to follow, no definite separation between these classes can be fixed, but in a general way it may be said that, where a destructive, rather than a propelling or pressure effect is required, the explosive used is one brought into operation by a concussive or detonating priming, and acting mostly by detonation within itself, such as dynamite, &c.

Whereas, up to nearly the middle of the nineteenth century, gunpowder was practically the only explosive in use for either civil or military purposes, the close of the century can show a list of several hundred preparations that have been proposed or actually used in its stead. The names by which these are put forward are expressive sometimes of an ingredient in their composition, such as “ammonia dynamite,” “cellulosa,” “mica powder,” “dynamite au carbon,” “dynamite de boghead,” &c.; and sometimes the inventor’s name, as “So-and-so’s powder or explosive”; sometimes of the strength of the mixture under various fanciful names, such as “dynamite,” “heraklin,” “vigorite,” &c., &c.; sometimes the names relate to the appearance of the compound, as “white gunpowder,” “blasting gelatine,” &c., &c.; and sometimes to other circumstances, such as “pudrolithe,” “saxifragine,” “safety powder,” &c., &c. A very long list might be given of the substances severally used in these various compositions. It will be sufficient to indicate the general nature of the several classes into which the new explosives may be divided. By turning back to p. 746, the reader will be reminded of the composition of gunpowder, and of the part played therein by the nitre (nitrate of potassium). Now a considerable number of the recently patented explosives are simply modified gunpowders, which all contain some nitrate, replacing wholly, or in part, the nitrate of potassium, while sulphur is an ingredient of nearly all, and in many, the charcoal of gunpowder is partly or wholly replaced by other carbonaceous materials, such as sawdust, coal-dust, tan, starch, paraffin, lycopodium, graphite, peat, flour, bran, &c. Certain mineral salts enter into the composition of some, such as sulphate of iron, carbonate of copper, sulphide of antimony, &c., &c.

In another class of the newer explosives chlorate of potassium takes the place of the nitrate as the oxygen supplying material, with similar variations as to the carbonaceous matter as are referred to above. Yellow prussiate of potash and sugar sometimes replace both the charcoal and sulphur of gunpowder in this class. Explosives of this chlorate class are usually dangerous to manufacture, and are often very sensitive, and also liable to changes by keeping, which render them still more dangerous.

The next class of preparations brings us to the “high explosives,” properly so called, and it is among these that most notable preparations are met with. Of all the explosive nitro-compounds, gun-cotton was the first practically employed (vide p. 741); but very soon after nitro-glycerine was discovered by Sobrero when working in Pelouze’s laboratory. This afterwards became known as “blasting oil,” but it was many years before nitro-glycerine came into use as an explosive, namely, when, about 1860, Nobel, a Swedish engineer, had established factories for its production as an agent for blasting. At first there were difficulties and dangers attending its use, and it was only when Nobel had discovered the detonation method of setting free its tremendous energy that the new era of “high explosives” really commenced. Between 1860 and 1870 such a number of appalling catastrophes occurred in the handling of the new “blasting oil” that in several European countries its use was entirely prohibited. And, in England at least, this prohibition remains, for “in a liquid state this explosive cannot be sold in, or imported into this country. It is manufactured under the strict provision that it is forthwith made up into dynamite or some kindred licensed explosive.” ... “The only source, practically speaking, of nitro-glycerine on a commercial scale in this country is the factory of Nobel’s Explosive Co. (Ltd.) at Ardeer, in the county of Ayr.”[17] Nitro-glycerine being so extremely dangerous to handle in the liquid form led Nobel to propose its use in an altered condition, by causing it to be absorbed by some inert porous material, the most suitable being a siliceous earth found in Germany, and there known as kieselguhr, of which one part will absorb three times its weight of liquid nitro-glycerine. Here we have the original dynamite, but now other substances are used for absorbing the liquid, and there are, indeed, dynamites of two different classes:

1. Dynamites with inert absorbents.

2. Dynamites with absorbents which are themselves combustible, or explosive.

Footnote 17:

Major Cundill, H.M.’s Inspector of Explosives.

Of the latter class there are endless varieties. One that has latterly been much used is called “blasting gelatine,” and is practically a combination of nitro-glycerine and nitro-cotton, this last ingredient being a less nitrated cellulose than gun-cotton. Blasting gelatine contains a very large percentage of nitro-glycerine (93–95 per cent.), and has the appearance of stiff jelly of a pale yellow colour. It may be of interest to remark that this second class of dynamites admits of well-defined sub-divisions according to the nature of the absorbent, as:

(a) Charcoal, or other simple carbonaceous material.

(b) Gunpowder, or other nitrate or chlorate mixtures.

(c) Gun-cotton, or other nitro-compounds.

MINERAL COMBUSTIBLES.

Certain mineral combustibles may fairly claim attention in a work treating of the discoveries of the nineteenth century, not because these bodies have been known and used only in recent times, but for other reasons. The true nature of coal—that most important of all combustibles—its relation to the past history of the earth, and to the present and future interests of mankind; the work it will do; the extent of the supply still existing in the bowels of the earth; the innumerable chemical products which it yields—are subjects on which the knowledge gained during the present century forms a body of discovery of the most interesting and important kind. Another substance we have to mention, though not a modern discovery, has lately been found in far greater abundance, and is now so largely used for various purposes, that it has become an important article of commerce.

COAL.

Most persons know, or at least have been told, that coal is fossil vegetable matter,—the long-buried remains of ancient forests. But probably many receive the statement, not perhaps with incredulity, but with a certain vague notion that it is, after all, merely a daring surmise. And, indeed, nothing is at first sight more unlike stems, or leaves, or roots of plants than a lump of coal. Then everybody knows that coal is found thousands of feet beneath the surface of the earth, whereas plants can grow only in the light of the sun. One begins to understand the matter only when the teachings of geology have shown him that, so far from the crust of the earth being, as he is apt to suppose, fixed and unchangeable, it is in a state of constant fluctuation. Changes in the levels of the ground are always going on: in one place it is rising, in another sinking; here a tract of land is emerging from the ocean, there a continent is subsiding beneath the water. The extreme slowness with which these changes proceed causes them to escape all ordinary observation. The case may be compared to the hour-hand on the dial, which a casual spectator might pronounce quite stationary, since the observation of a few seconds fails to detect its movement. As the whole period comprehended in human annals counts but as a second of geological time, it cannot be wondered at that it required a vast accumulation of facts, and much careful and patient deduction from them, before a conclusion was reached so apparently contradictory of experience. It is, indeed, startling to learn that “the sure and firm-set earth” is in a state of flow and change. Even the “everlasting hills” give evidence that their materials were collected at the bottom of the sea, and we know that the water which runs down their sides is slowly but surely carrying them back particle by particle. Of the magnitude of the changes which the surface of the earth has undergone in times past, and which are still imperceptibly but constantly proceeding, the ordinary experience of mankind can of itself give no example. But such changes have sufficed to entomb a vast quantity of relics of the innumerable forms of vegetation which flourished and waved their branches in the sun, ages upon ages before the advent of man.

It may be thought impossible that vegetable matter should have so changed as to become a dense, black, glistening, brittle mass, showing no obvious forms of leaves or texture of wood. But no one who has seen how a quantity of damp hay closely pressed together will, after a time, become heated and change in colour to black, can have any difficulty in comprehending how chemical and mechanical actions may completely alter the aspect of vegetable matter. We have, however, the most direct evidence of the vegetable origin of coal in the numberless unquestionable forms of trees and plants met with in all coal strata. Sometimes the trunks of the trees fossilized into stony matter are found upright in the very situation in which they grew. Thus in Fig. 341 is represented the appearance exhibited by the trunks and roots of some fossil trees, which were exposed to view in the formation of a railway cutting between Manchester and Bolton. In every coal-field also beautiful impressions of the stems and leaves of plants are met with—one common form of which is shown in Fig. 342. Most of the plants so found belong to the flowerless division of the vegetable kingdom. Some are closely allied to the ferns of the present day—to the common “mare’s-tail” (Equisetum), to the club-moss, and to other well-known plants. The firs and pines of the coal age are scarcely distinguishable from existing species. If a fragment of ordinary coal be ground to a very thin slice—so thin as to be transparent—and placed under the microscope, it will show a number of minute rounded bodies, which are, there is good reason to believe, nothing else than the spores or seeds of plants, closely resembling the existing club-mosses. The spores of the club-moss (Lycopodium) are so full of resinous matter, that they are used for making fireworks and the flashes of lightning at theatres. It is, therefore, extremely probable that the bitumen of coal is due to the resin of similar spores, altered by the effects of subterranean heat. The immense abundance of these little spores in the coal is a proof that they accumulated in the ancient forests as the mosses grew, and therefore the matter of coal was not accumulated under water or washed down into the sea; for these little spores are extremely light, and they cannot be wetted by water, and therefore they would have floated on the surface, and would not have been found so diffused throughout the coal. Fig. 343 is a picture of the possible aspect of the ancient forests of the coal age. In the humid atmosphere which probably prevailed at that period, the large tree-ferns and gigantic club-mosses, which are conspicuous in the picture, must have flourished luxuriantly.

The immense importance of coal for domestic purposes will be obvious from the fact that it is estimated that in the United Kingdom alone no less than 30,000,000 tons are annually consumed in house fires. Another great use of coal is in the smelting, puddling, and working of iron, and this probably consumes as much as our domestic fireplaces. Then there is the vast consumption by steam engines, by locomotives, and by steamboats. Another purpose for which coal is largely used is the making of illuminating gas; and to the foregoing must also be added the quantity which goes to feed the furnaces necessary in so many of the arts—such as in the manufacturing of glass, porcelain, salt, chemicals, &c. The quantity of coal raised in Great Britain was not accurately known until 1854, when it was ordered that a register should be kept, and an annual return made. The following figures, in round numbers, are the returns published up to 1873. The table is continued in Note A.

Year. Coal raised, in Tons. 1854 64,661,000 1855 64,453,000 1856 66,645,000 1857 65,395,000 1858 65,008,000 1859 71,979,000 1860 83,208,000 1861 85,635,000 1862 83,638,000 1863 88,292,000 1864 92,787,000 1865 98,150,000 1866 101,630,000 1867 104,500,000 1868 103,141,000 1869 107,427,000 1870 110,289,000 1871 117,352,000 1872 123,497,000 1873 127,017,000

The first return showed our annual produce to be 64,661,000 tons. The amount did not greatly vary until 1859, when there was an increased production of nearly seven millions of tons; in 1860 a further increase of eleven millions of tons more. Since then the quantity annually raised has been increasing. Comparing the quantity which has been raised in any year after 1863 with that raised ten years before, we see that the increase in ten years is nearly half as much again; or, that at the present rate of increase the amount annually raised doubles itself at least every twenty years. Now, the question arises, How long can this go on? However great may be the store of coal, it must sooner or later come to an end. Is it possible to calculate how long our coals will last? and what are the results of such calculations? These calculations have been made; but there are great discrepancies in the results, for the estimates of the amount of available coal still remaining vary greatly, and different views are held regarding the rate of consumption in the future. A very liberal estimate, by an excellent authority, of the quantity of coal remaining under British soil, makes it 147,000 millions of tons. With a consumption stationary at the present rate, this will last 1,200 years; with an increase of consumption of 3,000,000 tons a year, 276 years; but if the consumption continues to increase in the same geometrical ratio it has hitherto followed, the supply will scarcely last 100 years. It cannot be conceived, however, that this last will be the real case, for the increasing depth to which it will be necessary to go will soon cause a great increase in the cost, and thus effectually check the consumption. Great Britain will, however, be compelled to retire from the coal trade altogether, by the cheaper supplies which other countries will yield, long before the absolute exhaustion of her own coal-fields. It is calculated that the coal-fields of North America contain thirteen times as much as those of all Europe put together. Coal is also found abundantly in India, China, Borneo, Eastern Australia, and South Africa; and it is believed that these stores will supply the world for many thousand years.

Seeing, then, that our supply of coal has a limit, and that at the present increasing rate of consumption, the chief source of the wealth of Great Britain must necessarily be exhausted in a few more centuries, it behoves us to turn our mineral treasures to the best account, and to adopt every possible means of obtaining from our coal its whole available heat and force. The amount of avoidable waste of which we are guilty in the consumption of coal is enormous. This is especially the case in its domestic use, where probably nineteen-twentieths of the heat produced is absolutely thrown away—sent off from the earth to warm the stars. In England people look upon the wide open fireplace as the image and type of home comfort. No doubt there are, from long use and habit, many pleasing associations which cluster round the domestic hearth; but we, to whom it is given to “look before and after,” must think what it takes to feed that wide-throated chimney. All but a very small fraction of the heat thus escapes, and is lost to man and the world for ever; and surely we shall deserve the curses of our descendants if we continue recklessly to throw away a treasure which, unlike the oil in the widow’s cruse, is never renewed—for there is no contemporaneous formation of coal. Thanks to the enhanced price of coal during the last few years, some attention has been directed to contrivances for the economical consumption of coal in its domestic, as well as in its manufacturing, applications.

A time, however, will sooner or later come, when the whole available coal shall have been consumed. What will then be the fuel of the engines, and steamboats, and locomotives of the future? The reader may think that then it will only be necessary to burn wood. But wood is already being consumed from the face of the earth much more rapidly than it is produced. How, then, can it be available when coal fails? The truth is, we are now consuming not merely the wood which the sun-rays are building up in our own time, but in hewing down the forests we are using the sun-work of a century, while in coal we have the forests of untold ages at our disposal—the accumulated combustible capital stored up during an immense period of the earth’s existence. Upon this stored-up capital we are now living, our current receipts of sun-force being wholly inadequate to meet our expenditure. The coal is the sun-force of former ages; and it is from this we are now deriving the energy which performs most of our work. George Stephenson long ago declared that his locomotives were driven by sunshine—by the sunshine of former ages bottled up in the coal. And he was right. The mechanical energy of our steam engines, and the chemical energy of our blast furnaces, are derived from the combustion of vegetable matter, in which the heat and light of the sun—our present sun or that of the coal ages—are in some way stored up. The burning of wood or coal is, chemically, the reverse action to that performed by the sunlight: by the former carbon and oxygen are united, by the latter they are separated.

We foresee, then, a future period—however distant may be that future—in which the world’s capital shall have been exhausted, and the energies which are now employed in doing the world’s work will no longer be available. But the reader will perhaps think that by improvements in the steam engine, and in other ways, means will be found of getting more and more work out of coal. It is true that we obtain from coal only a fraction of its available energy; but the whole work which could, by any possible process, be done by the combustion of coal is definite and limited, although its amount is large. A pound of coal burnt in one minute sets free an amount of energy which would, if it could all be made available, do as much as 300 horses working in the same time. But, again, the reader may think, even if at some distant future the supplies of fuel for the steam engines of our remote posterity should fail, that before that time some other form of force than steam or heat engines will have been discovered—some application of electricity, for example. Now, it will appear, from principles which will be discussed in a subsequent article, that not only is there no probability of such a discovery, but that now, when the relations of the whole available energies of the globe have been traced and defined, Science can find no ground for admitting such a possibility under the present condition of the universe.

PETROLEUM.

When coal is heated in closed vessels, there are given off, as we shall presently see, a number of gaseous and volatile products—many being compounds of carbon and hydrogen—which condense to liquids or solids at ordinary temperatures. Carbon is by far the largest constituent of coal, which commonly contains only about 10 per cent. of other substances, although the proportions vary very widely. Another important constituent of coal is its hydrogen, and the value of coal as a source of heat depends almost entirely upon the carbon and hydrogen it contains. Carbon is one of the most remarkable of all the elements of the globe for its power of entering into an enormous number of compounds. Thus, for example, the compounds of carbon with only hydrogen are innumerable; but they are all definite, and their composition is expressible by the admirable system of chemical symbols, of which the reader has now, it is hoped, some definite notion. Perhaps these hydro-carbons are among the best evidences which could be adduced that modern science has obtained a grasp of certain conceptions which have a real correspondence with the actual facts of nature, even as regards the intimate constitution of matter. This is not the place to enter into a complete exposition of this subject. We may, however, invite the reader’s attention to a few simple facts. A very large number of compounds of carbon and hydrogen are known. If the percentage compositions of these be compared together, it is only the eye of a most expert arithmetician which can detect any relation between the proportions of the constituents in the various compounds. The chemist, however, by associating such of these compounds as resemble each other in their general properties, finds that they can be arranged in series, in which the composition is accurately expressed by multiples of the proportions: C = 12, H = 1. And not only so, the different series themselves form a series of series, having a simple relation to each other. Thus, confining ourselves to some of the known hydro-carbons, we have the following:

┌──────────────────────┬──────────────────────┬──────────────────────┐ │ A │ B │ C │ ├──────────────────────┼──────────────────────┼──────────────────────┤ │ C H_{4} │ C H_{2} │ │ │ C_{2}H_{6} │ C_{2}H_{4} │ C_{2}H_{2} │ │ C_{3}H_{8} │ C_{3}H_{6} │ C_{3}H_{4} │ │ C_{4}H_{10} │ C_{4}H_{8} │ C_{4}H_{6} │ │ C_{5}H_{12} │ C_{5}H_{10} │ C_{5}H_{8} │ │ C_{6}H_{14} │ C_{6}H_{12} │ C_{6}H_{10} │ │ &c. │ &c. │ &c. │ │ C_{n}H_{2_n_ + 2} │ C_{n}H_{2_n_} │ C_{n}H_{2_n_ – 2} │ └──────────────────────┴──────────────────────┴──────────────────────┘

┌──────────────────────┬──────────────────────┬──────────────────────┐ │ D │ E │ F │ ├──────────────────────┼──────────────────────┼──────────────────────┤ │ │ │ │ │ │ │ │ │ C_{3}H_{2} │ │ │ │ C_{4}H_{4} │ C_{4}H_{2} │ │ │ C_{5}H_{6} │ C_{5}H_{4} │ C_{5}H_{2} │ │ C_{6}H_{8} │ C_{6}H_{6} │ C_{6}H_{4} │ │ &c. │ &c. │ &c. │ │ C_{n}H_{2_n_ – 4} │ C_{n}H_{2_n_ – 6} │ C_{n}H_{2_n_ – 8} │ └──────────────────────┴──────────────────────┴──────────────────────┘

This table might be indefinitely extended, but enough is given to enable the intelligent reader to discover the laws connecting these formulæ. The series headed B, it will be observed, have all the same percentage composition. Why, then, one formula rather than another? The answer to this question is the statement of a theoretical law upon which the whole science of modern chemistry is based; for it has the same relation to that science as gravitation has to astronomy. It is a matter of fact that all gases, whatever their chemical nature, expand alike with the same application of heat, and all obey the same law, which connects volumes and pressures. These are very remarkable uniformities, for gases in this respect exhibit the most decided contrast to liquids and solids. The volume of each solid and of each liquid has its own special relations to temperature and pressure: here there is endless diversity. The volumes of all gases have one and the same relation to temperature and pressure: here there is absolute uniformity. As an explanation of these and other facts relating to gases, Amedeo Avogadro, in 1811, put forward this hypothesis—Equal volumes of all gases, under like circumstances of temperature and pressure, contain the same number of molecules. This hypothesis was revived by Ampère a few years later, and sometimes is called his. A necessary consequence of this law is that the weights of the molecules of gases are proportional to their densities or specific gravities. Hence when the percentage composition of a hydro-carbon has been determined, by burning or oxidizing it in such a manner as to obtain and weigh the products, carbonic acid and water, the next thing the chemist does is to obtain the weight of a volume of the gas. The number of times this exceeds the weight of hydrogen gas, under the same conditions, expresses how many times the molecule is heavier than the hydrogen molecule. Now, the chemist’s unit of weight in these inquiries is the weight of a single atom of hydrogen; and, as there are grounds for believing that the molecule of hydrogen consists of two atoms of that substance, its weight = 2. Now, if the molecule of marsh gas, the first hydro-carbon in the above list, has the composition assigned, it will be 12 + 4 = 16 times heavier than the atom of hydrogen, and 16/2 = 8 times heavier than the molecule of hydrogen. Hence, if Avogadro’s law be correct, marsh gas should be just eight times heavier than hydrogen gas; which is really the fact. The formula expressing the composition of the molecule of a hydro-carbon, or of any chemical compound whatever, is always so fixed that the same relations may hold; and almost the first thing a chemist does in examining a new body is to endeavour to obtain it in the state of gas.

The first four members of the series headed A are gases at ordinary temperatures, the fifth is a gas at temperatures above the freezing-point, and a liquid at lower temperatures; the next following members are liquids which boil (that is, are converted into gases) at temperatures rising with each additional carbon atom about 20° F. The specific gravities and boiling-points of these liquids augment as we pass from one hydro-carbon to the next, and the lower members of the series are solids, fusing at temperatures higher and higher as the number of carbon atoms is greater. Similar gradations of properties are exhibited by the other series of hydro-carbons. Petroleum or rock-oil is the name given to liquid hydro-carbons found in nature, and consisting chiefly of compounds belonging to the series marked A in the above list. Some varieties of petroleum hold in solution other hydro-carbons, and in some cases paraffin is extracted from the oils by exposing the liquid to cold, when the solid crystallizes out. Paraffin is a solid belonging to the B series, and it is for the most part obtained by heating certain minerals.

Deposits of liquid hydro-carbons, perhaps formed by a kind of natural subterranean distillation from coal or other fossil organic matter, exist in various localities. These deposits have long been known and utilized at Rangoon, in Burmah, and on the shores of the Caspian Sea. At Rangoon the mineral oil is obtained by sinking wells about 60 ft. deep in a kind of clay soil, which is saturated with it. The oily clay rests upon a bed of slate also containing oil, and underneath this is coal. It may be supposed that subterranean heat, acting upon the coal, has distilled off the petroleum, which has condensed in the upper strata. This petroleum, when distilled in a current of steam, leaves about 4 per cent of residue, and the volatile portion contains about one-tenth of its weight of a substance (paraffin) which is solid at ordinary temperatures. After an agitation with oil of vitriol, and another distillation, rock oil or naphtha is obtained, which, however, is still a mixture of several distinct chemical compounds. Mineral oils have also been found in China, Japan, Hindostan, Persia, the West India Islands, France, Italy, Bavaria, and England. In one of the Ionian Islands there are oil-springs which have flowed, it is said, over 2,000 years.

But it is the recently discovered and extremely copious springs and wells in Pennsylvania and Canada which have given a vastly extended importance to the trade in mineral oil. Rock oil is now used in enormous quantities as the cheapest illuminating oil, and that which furnishes the most intense light. Its consumption as a lubricating oil for machines has also been very large. Mineral oil was occasionally found at various places in the United States, and sometimes used by the inhabitants of the locality before the recent discoveries; but it was not until August, 1859, that it was met with in large quantities. About this time a boring which was made at Oil Creek, Pennsylvania, reached an abundant source, for 1,000 gallons a day were drawn from it for many weeks. The news of the discovery of this copious oil-spring spread rapidly: thousands of persons flocked to the neighbourhood in hopes of easily making a fortune by “striking oil.” Before the end of 1860 more than a thousand wells had been bored, and some of these had yielded largely. The regions of North America in which petroleum has been found cover a large part of the States, and comprise Pennsylvania, New York, Ohio, Michigan, Kentucky, Tennessee, Kansas, Illinois, Texas, and California. In the vicinity of Oil Creek the bore-holes are usually about 3 in. or 4 in. in diameter, and are often 500 ft. deep, and even 800 ft. is not uncommon. To make a bore-hole 900 ft. deep, and procure all the requisites—steam engines, barrels, &c., for pumping the oil—costs about $5,000. In 1869 many of these wells still yielded regularly 300 barrels a day, but the supply has not continued with the same abundance. One of the luckiest wells flowed at its first opening at the rate of about 25,000 barrels a day. The apparatus used for working the oil-wells is very simple—a rude derrick, a small steam engine, a pump, and some barrels and tubs being all that is necessary. Fig. 345 will give the reader an idea of the scene presented by a cluster of oil-wells in the Oil Creek region. Oil Creek received its name before the petroleum trade was established, from the oil found floating on the surface of the water. It is on the Alleghany River, about 150 miles above Pittsburg, and here at its mouth is situated Oil City. There is a wharf in Pittsburg for the oil traffic, and the barrels are brought down the river in flats, or the oil is poured into very large flat boxes, divided into compartments, which are then closed, and the boxes floated down in groups of twenty or more. The refining process consists in placing the crude oil into a large iron retort, connected with a condenser formed of a coil of iron pipes, surrounded by cold water. Heat is applied, and the lighter hydro-carbons (naphtha) come over first. After the naphtha, the oils which are used for illuminating purposes distil off. A current of steam is then forced into the retort, and this brings over the heavy oils which are used for greasing machinery. A black tarry oil yet remains; and, finally, after the separation of this, a quantity of coke. The products are subjected to certain processes of purification, which need not here be described. The magnitude of the American oil trade may be inferred from the fact that in the second year of its existence, from January to June, 1862, more than 4,500,000 gallons were exported from four seaports. This can hardly be wondered at, considering the extremely low price at which this excellent illuminating and lubricating agent can be produced. Refined petroleum can be bought at Pittsburg for 16 cents. per gallon. It is believed by some that the supplies of petroleum which exist in various localities are so abundant that they will furnish illuminating oils to the whole world for centuries.

PARAFFIN.

In the course of some researches on the substances contained in the tar, which is obtained by heating wood in close vessels, Reichenbach found a white translucent substance, to which he gave the above name, because it was not acted upon by any of the ordinary chemical reagents, such as sulphuric acid, nitric acid, &c. This substance, which is composed of carbon and hydrogen only, is not unlike spermaceti; it is colourless, translucent, and without smell or taste. But when slightly warmed, it becomes very plastic, and may then be moulded with the greatest ease—and in this respect it differs from spermaceti. Paraffin melts at from 88° to 150° C., to a colourless liquid, which is so fluid that it may be filtered through paper like water, and at a higher temperature it can be distilled unchanged. Paraffin does not dissolve in water, and is but slightly soluble in alcohol. In ether, naphtha, turpentine, benzol, and sulphide of carbon, it dissolves very readily. When heated with sulphur, it is decomposed: the sulphur seizes upon its hydrogen, sulphuretted hydrogen is given off, and the carbon is separated; and this action has been proposed as a ready means of obtaining pure sulphuretted hydrogen for laboratory use. It is probable that paraffin is a mixture of various hydro-carbons, having a composition expressible by the formula, C_{n}H_{2_n_}; for different specimens fuse at different temperatures, according as the paraffin has been obtained from one or the other source.

In the year 1847, Dr. Lyon Playfair directed the attention of Mr. James Young, then of Manchester, to a dense petroleum which issued from the crevices of the coal in a Derbyshire mine. It was soon found that this substance yielded a distillation—a pale yellow oil—which, on cooling, deposited solid paraffin. Mr. Young, recognizing the importance of this discovery, had an establishment at once erected on the spot, and the work of extracting paraffin was carried on until the supply of the petroleum had become nearly exhausted. Forced to seek for other sources of paraffin, Mr. Young was fortunate enough, after many trials, to discover that a species of bituminous coal, which occurs at Boghead, near Bathgate, in the county of Linlithgow, yielded by distillation annually large quantities of paraffin. In 1850 he procured a patent for “treating bituminous coals to obtain paraffin, and oil containing paraffin, therefrom.” This method consisted in distilling the coal in an iron retort, gradually heated up to low redness, and kept at that temperature until the volatile products ceased to come off. Under this patent, Mr. Young developed the manufacture of paraffin into a new and important branch of industry. The oil which first comes over in the distillation of the Boghead mineral is largely used for illuminating purposes under a variety of names besides that of paraffin oil, which term is, we believe, chiefly applied to a less volatile portion, extensively used for lubricating machinery, and consisting of liquid hydro-carbons of the same percentage composition as solid paraffin, which substance it also holds in solution. Mr. Young’s process consisted in placing the mineral in a retort encased in brickwork—an arrangement which caused the temperature of the retort to be more uniform than if the heat of the furnace had been applied to it directly. The retorts were placed vertically, and they were fed with the mineral by a hopper at the top. The products of the distillation passed through a worm tube surrounded by cold water into a cooled receiver. The result of the first distillation was a crude oily matter, differing from tar in being lighter than water, and in not drying-up when exposed to the air. This crude oil was then several times alternately treated with sulphuric acid and caustic potash, and distilled; and when about two-thirds of the oil had been separated from the rest, as an oil for burning and lubricating purposes, the residue yielded paraffin, or “paraffin wax,” as it is sometimes called. It is estimated that in Scotland no less than 800,000 tons of shale are annually distilled for mineral hydro-carbons, with a consumption of 500,000 tons of fuel. It is believed that about 25,000,000 gallons of crude oil are thus obtained, and from this 350,000 gallons of illuminating oil, 10,000 tons of lubricating oil, and 5,800 tons of solid paraffin are produced. Among the products exhibited in the International Exhibition of 1862, was a block of beautifully translucent paraffin, of nearly half a ton weight.

Paraffin is also obtained on the continent by distilling a variety of coal termed lignite. The tar which comes over is distilled, until nothing but coke remains. The condensed products are then treated with caustic soda, in order to remove carbolic acid and other substances. After washing with water, the oils are treated with sulphuric acid, in order to remove basic substances. The oil is again washed, and is then rectified by another distillation. The products which successively come over are, if necessary, separated by being collected in different vessels; but sometimes they are mixed together, and sent into the market as illuminating oils under various names, such as “photogen,” “solar oil,” &c. Oils having a specific gravity about 0·9 are collected apart, and are placed in tanks in a very cool place. In the course of a few weeks the solid paraffin, which is dissolved in the other hydro-carbons, crystallizes out. The liquid oils are drawn off, and the crude paraffin, which is of a dark colour, is freed from adhering oil by a centrifugal machine, and afterwards by pressure applied by hydraulic power. It then undergoes several other processes of purification before it is obtained as a colourless translucent solid.

Several thousand tons of paraffin are annually consumed for making candles, which is the most important application of the material. The variation in the fusing-points of different specimens is doubtless due to admixtures in greater or less proportion of other more easily fusible hydro-carbons. It was on account of the imperfect separation of these that the candles first made from paraffin were so liable to soften and bend, and felt greasy to the touch. Paraffin for candle-making is sometimes mixed with a certain proportion of other substances, such as palmitic acid, &c. Among the patented applications of paraffin are the lining of beer-barrels, and the preserving of fruits, jams, and meat. Some kinds of paraffin are also used in the manufacture of matches.

Liebig once expressed a wish that coal-gas might be obtained in a solid form: “It would certainly be esteemed one of the greatest discoveries of the age if any one could succeed in condensing coal-gas into a white, dry, odourless substance, portable and capable of being placed in a candlestick or burned in a lamp.” Now, it is curious that paraffin has nearly the same composition as good coal-gas: it burns with a bright and smokeless flame, and beautiful candles are formed of it, which burn like those made of the finest wax. When the fused paraffin first assumes the solid form, it is transparent like glass; and if it could be retained in that condition, we might have the pleasing novelty of transparent candles. But the particles seek to arrange themselves in crystalline forms, and the substance soon takes on its white semi-opaque appearance.

The great richness of the Boghead mineral in paraffin, which appears to exist in it ready formed, prevented for many years any successful competition by the working of other sources of supply. But paraffin is an abundant constituent of Rangoon petroleum, and considerable quantities may be obtained by distilling peat, and other fossil substances. All petroleums and paraffins are, in fact, mixtures of a number of hydro-carbons, which in many cases cannot be entirely separated from each other. The accidents which have from time to time occurred with some of these combustibles, and have caused legislative enactments with regard to them, are due to the imperfect removal by distillation of the more volatile bodies, which rise in vapour at ordinary temperatures. Explosions of the hydro-carbons can occur only under the same circumstances as with coal-gas; that is to say, the application of a flame to a mixture of the vapour with atmospheric air.

COAL-GAS.

When coal is burning in a common fire, we may see jets of smoky gas issuing from the pieces of coal before they become red hot. This vapour, coming in contact with flame in another part of the fire, may often be observed to ignite, thus supplying an instance of gas-lighting in its most elementary form. In the ordinary fire the air has free access, and the inflammable gases and vapours continue to burn with flames more or less bright, and when these have ceased the carbonaceous portion continues afterward to glow until nearly the whole has been consumed, except the solid residue which we call the ashes. These ashes in general contain a portion of unconsumed carbon, mixed with what is chemically the ash, namely, certain incombustible salts, constituting the white part of the ashes. If, however, we heat the coal in a vessel which prevents access of air, and allows the gases to escape, the coal is decomposed much in the same way as when it is burnt in the open fire; but the products formed are no longer burnt, the supply of oxygen being cut off. Every one knows the familiar experiment of filling the bowl of a common clay tobacco-pipe with powdered coal, then covering it with a dab of clay, and placing it in a fire. The gas which soon comes from the stem of the pipe does not take fire unless a light be applied, when it may be seen to burn with a bright flame, and after the flow of gas has ceased, nearly the whole of the carbon of the coal will be found unconsumed in the bowl of the pipe. This simple experiment illustrates perfectly the first step in the manufacture of coal-gas, namely, the process of heating coal to redness in closed vessels, by which operation the substances originally contained in coal are destroyed, and their elements enter into new combinations.

These elements are few in number; for, except the very small portion which remains as incombustible white ash, coal is constituted of carbon, hydrogen, oxygen, nitrogen, and a little sulphur. All the varied and interesting products obtained by the destructive distillation of coal are combinations of two or more of these four or five elements. Illuminating gas is far from being the only product when coal is heated without access of air; for of the numerous substances volatized at the red heat of the gas-retort a great number are not only incapable of affording light, but liable to generate noxious compounds when burnt. Besides this there are numerous bodies which, though leaving the retort in the gaseous form, immediately assume the liquid or solid state at ordinary temperatures. All such substances must be separated before permanent gases are obtained fit for illuminating purposes and capable of being carried through pipes to distant places. Thus an important part of the apparatus for gas manufacture consists in arrangements for separating the condensable bodies, and for removing useless or injurious gases from the remainder.

The products resulting from the destructive distillation of the coal may, therefore, be classified as—a, solids left behind in the retort; b, solids and liquids condensed by cooling the vapours which issue from the retort; c, coal-gas—a mixture of gases from which certain useless and noxious constituents must be removed. Fig. 347 is intended to give a diagrammatic view of the apparatus employed in the generation, purification, and storage of gas, the various parts being shown in section. A is the furnace containing several retorts, of which B is one. From each retort a tube, d, rises vertically, and curving downward like an inverted U, it enters a long horizontal cylinder, f; half filled with water, beneath the surface of which the open end of the recurved tube dips. The cylinder containing water passes horizontally along the whole range of furnaces in the gas-works, and is known as the hydraulic main. It is here, then, the tar and the moisture first condense, and the pipe is always kept half full of these liquids, so that the ends of the pipes, d, from the retorts, dipping beneath its surface, form traps or water-valves, which allow any retort to be opened without permitting the gas to escape. As the tar accumulates in the hydraulic main, it flows over through a pipe, g, leading downwards into the tar-well, H. The gases take the same course; but while the tar flows down the vertical tube, R, the gases pass on through j into the condensers or refrigerators. Gas cannot escape from the open end of the tube, for it is always closed by the liquids—tar and ammoniacal liquor—which accumulate and flow over the top of the open inner vessel into the cistern, S, from which they are drawn off from time to time by the stop-cock, I. Although when the gas has arrived at this cistern much of its tar and ammoniacal vapours have been condensed, a portion is still retained by reason of the high temperature of the gas; and this has to be removed before it is permitted to enter the purifier. This is the object of passing the gas through the series of pipes, j j, forming the condenser. These are kept cool by the large surface they expose to the air, and, when necessary, cold water from the cistern, K, may be made to flow over them. The tar and other liquids condense in the iron chest, T, which is so divided by partitions as to compel the gas to pass through the whole series of tubes; and as the liquid accumulates, it also overflows into the tar-well. The cooled gas then enters the purifier, L L, in which are layers of slaked lime placed on a number of shelves. By contact with the extensive surface of slaked lime the gas has its sulphuretted hydrogen, carbonic acid, and some other impurities, removed; and it then, through the tube n, enters the gasholder, in which it is stored up for use.

Hydrated oxide of iron is now much used for purifying coal-gas. The oxide is mixed with sawdust, and placed in layers 10 in. thick. Sulphide of iron and water is formed; and when the mixture has ceased to absorb any more, it is removed and exposed to a current of air; the hydrated oxide is thus reproduced and sulphur set free. The process may be repeated many times in succession, until the absorbent power is impaired by the accumulation of sulphur.

The gasholder—or “gasometer,” as it is often improperly named—is an immense cylindrical bell, made of wrought iron plates, and inverted in a tank of water, in which it rises or falls. It is counterpoised by weights attached to chains passing over pulleys, so as to press the gas with a small force in order to drive it along the main, which communicates with the pipes supplying it to the various consumers. The pressure impelling the gas through the mains does not in general exceed that of a column of water two or three inches high.

It will be necessary, after this slight outline describing the essential parts of the apparatus, to enter more fully into the details of the several parts.

The retorts are constructed of wrought iron, cast iron, or earthenware, and in shape are cylindrical, with a diameter of 12 in. to 18 in., or more, and a length of 6 ft. to 10 ft. Though sometimes circular in section, other forms are commonly used—such as the elliptical, and especially the ⌓-shaped. The retorts are closed except at the mouth-end, Fig. 348, from the top of which rises the stand-pipe, A, which has usually a diameter of about 5 in. When the charge has been introduced, the mouth is closed by a plate of iron, B, closely and securely applied by means of a screw, C, as shown in the figure—a perfectly tight joint being obtained by a luting of lime mortar spread on the part of the lid which comes into contact with the mouth of the retort. The retorts are always set horizontally in the furnace—each furnace usually including a set of five retorts. The charge of coals is introduced on a tray of sheet iron adapted to the size of the retort, which, when properly pushed in, is inverted so as turn out the contents, and then withdrawn.

The time required to completely expel the volatile constituents from the charge in a gas retort varies very much, because there are great diversities in the composition of the different kinds of coal employed. Some varieties of coal, such as cannel, are easily decomposed, and the operation may be complete in about three hours; while other kinds may require double that time. The quantity of gas procurable from a given weight of coal also varies according to the kind of coal made use of. Thus, while a hundredweight of cannel may give 430 cubic feet of gas, the same weight of Newcastle coals will yield but 370 cubic feet. The nature of the gases given off from a retort will be different at the different stages of the operation.

The scene presented by the retort-house of a large gas manufactory, when viewed at night, is a singular spectacle. The strange lurid gleams which shoot out amid the general darkness as the retorts are opened to withdraw the coke, and the black forms of the workmen partially illuminated by the glare, or flitting like dark shadows across it, form a picture which might engage the pencil of a Rembrandt. In Fig. 348_a_ is depicted the retort-house at the Imperial Gas Works, King’s Cross. Here the retorts are arranged in several tiers—the coal being brought, and the coke withdrawn, by the aid of an iron carriage running on rails parallel to the line of furnaces.

In the process of heating, a proper regulation of the temperature is of the highest importance. It is found that when the retorts are heated to bright cherry-red, the best results are obtained. At a lower temperature a larger quantity of condensable vapours are given off, which collect in the gasholders and distributing pipes as solid or liquid, and occasion much inconvenience, while the quantity of gas obtained is decreased. On the other hand, if the temperature be too high, some of the gases are decomposed, and the quantity of carbon contained in the product is so much diminished as seriously to impair the illuminating power. Again, every second the gases after their production remain in the red-hot retort diminishes their light-giving value; for those hydro-carbons on which the luminiferous power of the gas depends, are then liable to partial decomposition; a portion of their carbon is deposited on the walls of the retort in a dense layer, gradually choking it up, while the liberated hydrogen does not add to the illuminating but to the heating constituents of the gas. A plan has been patented by Mr. White, of Manchester, for rapidly removing the illuminating gases from the retort by sweeping them out by means of a current of what has been termed “water gas.” This water gas is produced by causing steam to pass over heated coke, and is a mixture of carbonic acid, carbonic oxide, and hydrogen. Though only two of these are combustible gases—and even they do not yield light by their combustion, and, by adding to the bulk of the gas, serve rather to dilute it—yet it has been found that in some cases twice the amount of light is obtainable by White’s process than the same weight of coal supplies when treated in the ordinary manner.

The hydraulic main, as already mentioned, being kept half full of tar into which the lower ends of the dip-pipes descend, prevents the gas from escaping through the stand-pipes when the lid of a retort is removed for the introduction of a fresh charge. The hydraulic main is from 12 to 18 in. diameter, and the dip pipes pass into it by gas-tight joints. Various forms of purifiers are in use besides the simple one already mentioned. Some of these have arrangements for agitating the gas with a purifying liquid by mechanical means, the motion being supplied by a steam engine.

The gasholder, as it sinks in the water of the cistern, presses with less force on the contained gas, and unless this inequality of pressure were counteracted there would be very unequal velocities in the flow of gas from the burner. The equality of pressure is obtained by making the weight of the chains by which the gasholder is suspended equal to half the weight the gasholder loses in the same length of its motion. Gasholders are also constructed without chains or counterpoises, as these are found to be unnecessary where the height of the gasholder does not exceed half its width. In such cases, especially when the vessel is very large, the difference of pressure at the highest and lowest position is quite inconsiderable, and nothing more is necessary than that upright guides or pillars be placed to preserve the vertical motion of the vessel. Another improvement, which enables a lofty gasholder to be used without increasing the depth of the tank, consists in forming the gasholder of several cylinders, which slide in and out of one another like the draw-tubes of a telescope. Each cylinder has a groove formed by turning up the iron inside the rim, and at the top of the next cylinder the edge is turned outwards so as to drop in the groove or channel, which thus forms a gas-tight joint, for it is of course filled with water as it rises. The pressure is, however, more accurately regulated by an apparatus called the governor, through which the gas passes in before it enters the mains. The construction and action of the regulator will be understood from Fig. 349, where A represents a kind of miniature gasholder, inverted in the cistern, B. From the centre of the interior of the bell hangs a cone, C, within the contracted orifice of the inlet-pipe. If this cone be drawn up, the size of the orifice, D, is reduced, and, on the other hand, by its descent it enlarges the opening through which the gas passes outward. By properly adjusting the weights of the counterpoise, E, such a position of the cone may be found that the gas passes into the mains at an assigned pressure. Suppose, now, that from any cause the pressure of gas in F increases, that pressure acting upon the inverted bell, A, causes it to rise and carry with it the cone, which, by narrowing the orifice of the outlet, checks the flow of gas. Similarly, a decrease of pressure in the mains would be followed by the descent of the cone, and consequently freer egress of gas. In hilly towns it is necessary to fix regulators of this kind at certain heights in order to equalize the pressure. It is found that a difference of 30 ft. in level affects the pressure of gas in the same main to about the same amount as would a column of water one-fifth of an inch high, the pressure being least at the lowest point.

Coal-gas is a mixture of several gases, and these may be classified as, first, the light-giving gases, or those which burn with a luminous flame; secondly, gases which burn with a non-luminous flame, and which therefore contribute to the heat, and not to the light, of a gas-flame, and have the effect of diluting the gas; third, gases and vapours which are properly termed impurities, as they are either incombustible or by their combustion give rise to injurious products. Of the first kind the principal is olefiant gas, a gas which burns with a brilliant white flame without smoke. It is a compound of hydrogen and carbon, six parts by weight of carbon being combined with one part by weight of hydrogen. Besides olefiant gas other gaseous hydro-carbons are found in smaller quantities. These contain a larger proportion of carbon than olefiant gas. The second class contains hydrogen, light carburetted hydrogen, and carbonic oxide. Hydrogen is one element of water, of which it forms one-ninth of the weight. It burns with a flame giving singularly little light, but having intensely heating power; in fact, one of the brightest lights we can produce is obtained by allowing the flame of burning hydrogen to heat a piece of lime. Light carburetted hydrogen, like olefiant gas, is a compound of hydrogen and carbon, but the proportion of carbon to hydrogen is only half what it is in olefiant gas, namely, three parts to one. This gas enters largely into the composition of coal-gas, and occurs naturally in the coal seams, being, in fact, the dreaded fire-damp of the miner. It is much lighter than olefiant gas, for while that gas is of nearly the same specific gravity as atmospheric air, light carburetted hydrogen is only a little more than half that specific gravity. It is this ingredient of coal-gas which renders it so light as to be available for inflating balloons. It burns with either a bluish or a slightly yellow flame, yielding hardly any light. Olefiant gas and the other luminiferous hydro-carbons, when exposed to a bright red heat, split up for the most part into this gas and carbon. This explains the importance of rapidly removing the gas from the retort in which it is generated, a point which has been referred to above. Carbonic oxide is a gas which one may often see burning with a pale blue flame above the glowing embers of a common fire, the flame giving, however, little light. It is a compound of carbon and oxygen, containing only one-half the quantity of oxygen which its carbon is capable of uniting with, and therefore ready to unite with another proportion, which it does in burning, carbonic acid being the product.

The third class of constituents of coal-gas—the impurities—are those which the manufacturer strives to remove by passing the gas over lime, milk of lime, oxide of iron, &c. Sulphuretted hydrogen, a compound of sulphur and hydrogen, has an extremely nauseous odour resembling that of rotten eggs. It is always formed in the distillation of coal, and if not removed from the gas in the process of purification, it has a very objectionable effect; for one product of its combustion is sulphurous acid, and in a room where such gas is burnt much damage may be done by the acid vapours; for example, the bindings of books, &c., soon become deteriorated from this cause. The detection of sulphuretted hydrogen in coal-gas is quite easy, for it is only necessary to hold in a current of the gas a piece of paper dipped in a solution of the acetate of lead. If in a few minutes the paper becomes discoloured the presence of sulphuretted hydrogen is indicated.

But the bête noire of the gas-maker is a substance called “sulphide of carbon,” which is formed whenever sulphur and carbonaceous matters are brought together at an elevated temperature. Sulphide of carbon is, in the pure state, a colourless liquid, of an intensely offensive odour, resembling the disagreeable effluvia of putrefying cabbages. The liquid is extremely volatile, and coal-gas usually contains some of its vapour. When too high a temperature is used in the generation of the gas, it contains a large quantity of this deleterious ingredient, especially if the amount of sulphur contained in the coal is at all considerable. This sulphide of carbon vapour is very inflammable, and one product of its combustion is a large quantity of sulphurous acid. This substance cannot be removed from coal-gas by any process sufficiently cheap to admit of its application on the large scale. It is said, however, that by passing the gas over a solution of potash in methylated spirit, the sulphide of carbon vapour can be completely got rid of. The price of these materials renders the process available in special cases only, where the damage done by the sulphurous acid would be serious, as in libraries, &c. Besides the impurities we have already enumerated, many others are present in greater or less quantity. Carbonic acid—the gas resulting from the complete combustion of carbon—should be entirely removed by the lime purifiers, as the presence of even a small percentage detracts materially from the illuminating power. This gas is not inflammable and cannot support combustion. It has decided acid properties, and readily unites with alkaline bases forming carbonates: it is upon this behaviour that its removal by lime depends. The illuminating power of coal-gas containing only 1 per cent. of carbonic acid is reduced thereby by about one-fifteenth of its whole amount.

The proper mode of burning the gas so as to obtain the maximum amount of light it is capable of yielding requires a compliance with certain physical and chemical conditions. The artificial production of light depends upon the fact that by sufficiently heating any substance, it becomes luminous, and the higher the temperature the greater the luminosity. The light emitted by solid bodies moderately heated is at first red in colour; as the temperature rises it becomes yellow, which gradually changes to white when the heat becomes very intense. The widest difference exists, however, in the temperature required to render solids or liquids luminous, and that needed to cause gases to give off light. In all luminous flames the light is emitted by solid particles highly heated. Every luminous gas-flame contains solid particles of carbon, as may be easily shown by the soot deposited on any cold body—such as a piece of metal—introduced into the flame. On the other hand, the flame of burning hydrogen, which produces only aqueous vapour, furnishes no light, but a heat so intense, that a piece of lime introduced into the jet becomes luminous to a degree hardly supportable by the eye. The conditions requisite, therefore, for burning illuminating gas are, first, just such a supply of air as will prevent particles of carbon from escaping unconsumed in the form of smoke, and yet not enough to burn up the carbon before it has separated from the hydrogen, and passed through the flame in the solid state; second, the attainment of the highest possible temperature in the flame, compatible with the former condition. When the supply of oxygen is not in excess, the hydrogen of the gaseous hydro-carbon appears to burn first; the carbon is set free, and its solid particles immersed in the flame of the burning hydrogen are there intensely heated; but ultimately reaching the outer part of the flame, they enter into combination with the oxygen of the air, producing carbonic acid; or if present in excessive quantity, they are thrown off as smoke. If the purpose of burning the gas is to obtain heating effects only, this is accomplished by supplying air in such quantities, that the carbon enters into combination with oxygen in the body of the flame, without a previous separation from the hydrogen with which it is combined. In this case a higher temperature is attained, and the flame is wholly free from smoke; so that vessels of any kind placed over it remain perfectly clean and free from the least deposit of soot. The last result is of great advantage in chemical processes, especially where glass vessels require to be heated, for the chemist retains an uninterrupted view of the actions taking place in his flasks and retorts.

No better illustration of the nature of the combustion in a gas-flame can be found than is furnished by Bunsen’s burner, Fig. 350, now universally employed as a source of heat in chemical laboratories. In this burner the gas issues from a small orifice at the level of a, near the bottom of the tube, b, which is open at the top, and is in free communication at the bottom with openings through which air enters and mixes with the gas, as they rise together in the tube and are ignited at the top. If the pressure of the gas be properly regulated, the flame does not descend in the tube, but the mixture burns at the top of the tube, producing a pale blue flame incapable of emitting light, but much hotter than an ordinary flame, for the combustion is much quicker. If the openings at a be stopped, the supply of air to the interior of the tube is cut off, and then the gas burns at the top of the tube, b, in the ordinary manner, giving a luminous flame. Ordinary gas-jets burning in the streets, at open stalls or shops, may be seen on a windy night to have their light almost extinguished by the increased supply of oxygen, carried mechanically into the body of the flame, the white light instantly changing to pale blue. The disappearance of the light in such cases is due, as in Bunsen’s burner, to the supply of oxygen in sufficient quantity to combine at once with the carbon as well as the hydrogen of the hydro-carbons.

The burners now chiefly used for the consumption of coal-gas for illuminating purposes are the bat’s-wing, the fish-tail, and various forms of Argand. The bat’s-wing burner is simply a fine slit cut in an iron nipple, and it produces a flat fan-like flame. The fish-tail is formed by boring two holes so that two jets of gas inclined at an angle of about 60° infringe on each other and produce a flat sheet of flame. The Argand, in its simplest form, consists of a tubular ring perforated with a number of small holes from which the gas issues. Many modifications of this kind of burner have been devised, in all of which a glass chimney is requisite to obtain a current of air sufficient to consume the gas without smoke, and it is important that the height of the chimney should be adapted to the amount of light required if the gas is to be used economically. Argand burners are specially advantageous where a concentrated light is required. Fig. 351 represents a ventilating gas-burner, contrived by Faraday, the object being to remove from the apartment the whole of the products of the combustion of the gas. A is the pipe conveying the gas to the Argand burner, B, the flame of which is enclosed in the usual cylindrical glass chimney, C C, open at the top. This is enclosed in a wider glass cylinder closed at the top by a double disc of talc, D D, and opening at its base into the ventilating tube, E E. The direction of the currents produced by the heat of the flame is shown by the arrows. The whole is entirely enclosed by a globe of ground glass. Means are provided for regulating the draught in the pipe, E E, which, when heated, creates of itself a strong current of air through the apparatus.

The illuminating power of coal-gas may be measured directly by comparing the intensity of the light emitted by a gas-flame consuming a known quantity of gas per hour with the light yielded by some standard source. The standard usually employed is a spermaceti candle burning at the rate of 120 grains of sperm per hour. It is not necessary that the candle actually used should consume exactly this amount, but the consumption of sperm by the candle during the course of each experiment is ascertained by the loss of weight, and the results obtained are easily reduced to the standard of 120 grains per hour. An instrument is used for determining the relative intensities of the illumination, called Bunsen’s photometer. It consists of a graduated rule, or bar of wood or metal, about 10 ft. long. At one end of this bar is placed the standard candle, at the other is the gas-flame. A stand slides along the rule supporting a circular paper screen at the level of the two flames, and at right angles to the line joining them. This paper screen is made of thin writing-paper, which has been brushed over with a solution of spermaceti, except a spot in the centre, or, more simply, a grease-spot is made in the middle of a piece of paper. In consequence the paper surrounding the spot is much more transparent; yet when it is placed so that both sides are equally illuminated, a spectator will not perceive the spot in the centre when viewing the screen on either side. When the screen has been placed by trial in such a position between the two sources of light, it is only necessary to measure its distance from each flame in order to compute the number of times the illuminating power of the gas-flame exceeds that of the candle. This computation is based on the fact that the intensity of the light from any source diminishes as the square of the distance from the source. Thus, if a sheet of paper be illuminated by a candle at 2 ft. distance, it will receive only one-fourth of the light that would fall upon it were its distance but 1 ft., and if removed to 3 ft. distance it has only one-ninth of the light. In the instrument used for measuring the illuminating power of gas the rule is graduated in accordance with this law, so that the relative intensities may be read off at once. The gas passes through a meter for measuring accurately the quantity per minute which is consumed by the burner, and there is also a gauge for ascertaining the pressure. Another mode of estimating the illuminating power of coal-gas is by determining the quantity of carbon contained in a given volume. For, in general, the richness of the gas in carbon is a fair index of the quantity of its luminiferous constituents. This may be readily effected by exploding the gas with oxygen, and measuring the amount of carbonic acid produced. Still more accurate determinations of the illuminating value of gas may be obtained by a detailed chemical analysis.

The illuminating power of any gas is so calculated that it represents the number of times that the light emitted by a jet of the gas, burning at the rate of 5 cubic feet per hour, exceeds the light given off by the standard sperm candle burning 120 grains of sperm per hour. For example, when it is said that the illuminating power of London gas is 13, it is meant that when the gas is burnt in an ordinary burner at the rate of 5 cubic feet per hour, the light is equal to that given by thirteen sperm candles burning together 13 × 120 grains per hour. The quality of gas varies very much, as it depends upon the kind of coal employed, and upon the mode in which the manufacture is conducted. The following are the results of experiments made to determine the illuminating power of the gas supplied to several large towns:

Candles. London 12·1 Paris 12·3 Birmingham 15·0 Berlin 15·5 Carlisle 16·0 Liverpool 22·0 Manchester 22·0 Glasgow 28·0

The relative quantities of tar, ammonia water, and coke yielded in various gas manufactories also vary very considerably for the same reasons.

In the early days of gas illumination the consumers were charged according to the number of burners; but this arrangement proved so unsatisfactory that the gas-meter became a necessity, and already in 1817 meters had been devised, which were not essentially different from those now in use. Although gas is used in so many houses, there are few persons who have any notion of the mechanism of the gas-meter. Our space will not allow full details of the construction, but the following particulars may be mentioned. In the ordinary “wet” meter there is a drum divided into four compartments by radiating partitions; this drum revolves on a horizontal axis, and the lower half of the drum, or rather more, is beneath the surface of water contained in the case, the water being at the same level inside and outside the drum. The gas enters one of the closed chambers formed between the surface of the water and a partition of the drum. Its pressure tends to increase the size of the chamber, hence the drum revolves. The preceding division of the drum being filled with gas, this is driven into the exit pipe by the motion of the drum, as it is included in a space comprised between the water and a partition. Each division in turn comes into communication with the gas-main, and as it is filled passes on towards the position in which a passage is opened for it to the exit-pipe. Each turn of the drum, therefore, carries forward a definite quantity of gas, and the only thing necessary is a train of wheels, to register the number of revolutions made by the drum. The “wet” meter is much inferior in almost every respect to the “dry” meter, in which no water is used. The principle of the “dry” meter is very simple. The gas pours into an expanding chamber, partly constructed of a flexible material, and which may be compared to the bellows of a circular accordion. The expansion is made to compress another similar chamber, already filled with gas, which is thus forced through the exit-pipe. When the first chamber has expanded to a definite volume, it moves a lever, and this reverses the communications. The expanded chamber is now opened to the exit-pipe, and the other to the entrance-pipe, and so on alternately. A train of wheels registers the number of movements on a set of dials.

Recent years have brought no essential changes in the methods of gas making, although of course improvements in many minor details of the processes and of the apparatus have been effected. These demand no description at our hands, as they are of interest only to those concerned with the actual technology of gas-making, nor need some of the later forms of burners for using the gas be noticed, as these are sufficiently familiar. They really do effect a considerable economy in the consumption of gas, especially in cases where a more powerful light is required. But the reader will have already learnt from a foregoing section on Electric Lighting that the importance of gas as an illuminant is already on the wane. Indeed, it will not be too much to say that, before the close of the present century, every town will have its streets, and still more certainly, all its places of public assembly, such as theatres, concert halls, churches, libraries, &c., fitted with installations for electric illumination, and even in shops and private houses, it is probable that before long, gas will be superseded by the electric light. Some of the disadvantages of burning gas have already been referred to, and the danger attending its accidental escape into apartments is illustrated by the yearly tale of victims to suffocation and violent explosions. The inherent disadvantage of gas used as an illuminant, is the enormous quantity of heat produced by its combustion, compared with the amount of light evolved. The absolute quantity of heat required to render a body highly luminous is really very small, for masses of matter almost inappreciable become very luminous, provided only that their temperature be sufficiently raised. Thus, for example, the few residual particles of gas in a Geissler’s tube (p. 431) become incandescent by electrical discharges, while the number of them is too small to sensibly heat the glass vessel, and the very attenuated carbon filament in an electric glow lamp suffices by the mere contraction and concentration of the current within it raising its temperature high enough, to diffuse a brighter light than a large gas-flame. This explains the fact alluded to elsewhere, that if instead of burning the gas we use it in a gas engine, driving a dynamo connected with an electric light installation, we shall obtain a much greater luminous effect. As there is no combustion, the surrounding air is neither heated nor deteriorated with gaseous products and smoke.

Without any rivalry from the electric-light, gas, as a domestic luminant, has now met with a competitor on the ground of cheapness in the mineral oils mentioned in the preceding article. If these could be deprived of their unpleasant odour, and a perfectly safe lamp contrived for burning them, it would be only under very favourable conditions that gas could compete with them on the score of economy. But of late years two applications of gas to other purposes than to illumination will have been observed. First to heating, for warming, cooking, and other domestic purposes, and also in various processes in the arts. In all the appliances so used, the principle of Bunsen’s burner (p. 722) is adopted, and stoves, fireplaces, and kitchen-ranges, heated by gas have obvious advantages in their greater cleanliness and readiness. The other new application of gas is as a motive power in the gas engine, by which a very convenient supply of mechanical energy is afforded. There can be little doubt that in the future, gas will be greatly used for these purposes, and perhaps be for them consumed as largely as at present. A singular thing in the history of gas-manufacture is the great value that the bye-products have attained, that is to say, the ammoniacal liquor, the coke, and especially the tar. So many valuable substances are now derived from this last, that even if coal should cease to be destructively distilled for gas, the operation would still be largely carried on if only for the tar.

A jet of hydrogen gas burning in a dark room is all but invisible, yet no gas can give so intense a heat. The lime-light, which no doubt is perfectly familiar to everyone as an illuminant in magic lantern projections, is simply a jet of mixed hydrogen and oxygen gases directed on a piece of lime, which is rendered incandescent by the heat. The flame of the Bunsen burner (p. 772) is distinguishable only by a very pale blue colour, and it is impossible to discern objects, or to read by its light in an otherwise dark room. But if a piece of thin platinum wire formed into a coil, as by twisting round a pencil, be introduced into the flame, the wire will glow with great brilliancy, and its thickness will seem much increased. It will, in fact, emit so much light that reading by its glow becomes easy. This shows that, as already stated, a solid will give off light at a temperature which scarcely suffices to make a gas visible. Thus a Bunsen burner flame can be made to give light simply by putting into it some incombustible solid, which itself incapable or suffering any chemical change under the conditions, nevertheless becomes luminous by merely acquiring the temperature of the almost invisible heated gas. The cause of the luminosity of the ordinary gas burner, as compared with the almost invisible Bunsen burner flame, has, indeed, been already explained on a previous page, but the phenomenon is again, by the experiment just referred to, brought clearly before our attention; and it becomes obvious that substances other than the carbon of the hydro-carbon constituents of the coal gas will emit rays of light. Chemical analysis shows that by far the larger proportion of the constituents of ordinary coal-gas consist of gases which do not themselves produce luminous flames, and that, taking 16 candle-gas, about 10 candles of the illuminating power is due to compounds of which the gas does not contain more than 4 per cent. Nearly half the bulk of purified coal-gas is hydrogen, which itself gives no light whatever when burnt; marsh-gas, which burns with only a slight luminosity, forms 35 per cent. of ordinary coal-gas; and there is usually present about 7 per cent. of carbonic oxide, which in burning gives only a pale blue flame. This shows that by far the greatest product of the combustion of coal-gas is not light but heat. The flame of hydrogen is much the hottest known, and as that gas enters so largely into the composition of coal-gas, and the complete combustion of all the other constituents takes place when the gas is previously mixed with air, as in the Bunsen burner, we are provided with an economical means of obtaining high temperatures. But coal-gas was in the first instance intended to provide us with a cheap illuminant, and although for some time the gas itself was very impure, and it was long before the crude appliances for burning it were superseded by contrivances giving steadier and more brilliant lights, such as the Argand and the regenerative burners. It is only quite recently that the full illuminating possibilities of coal-gas have been developed by the happy notion of converting the heating power of its flame into light-giving power, by the simple plan of suspending a suitable solid over the hot but non-luminous Bunsen burner.

The manner in which an effective method of doing this was discovered is not a little curious. The construction of the ordinary incandescent electric lamp, Fig. 280_h_, involves the necessity of enclosing the carbon filament in an exhausted glass bulb; and it was when Auer von Welsbach was engaged in attempting to find some substance that could be brought into incandescence by the electric current, and yet be incombustible even in the open air, that his investigations led to the invention we have now to describe—an invention apparently destined to give a new lease of life to coal-gas illumination.

It is singular also that Welsbach, in seeking for the most suitable materials for heating to incandescence in the Bunsen burner flame, should find them in certain very rare minerals, containing a group of elements formerly of interest only to the scientific chemist, and up to that time devoid of any practical applications. The names of these elements, the oxides of which are called “earths,” will, of course, be strange to non-chemical readers, but we give their names, with the remark that the nearest familiar substance they at all resemble is aluminium, of which the oxide, or “earth,” is alumina. These rare metals, the oxides of which are the materials of the Welsbach “mantle,” are all discoveries of the present century, or nearly so, and they are called lanthanum, zirconium, thorium, cerium, didymium, yttrium, erbium, &c. They occur as silicates or phosphates very sparingly, and in a few localities in Norway; but some of them have now been found more abundantly in America. The minerals, from which for the most part the oxides are obtained, are called monazite, orthite, and thorite. It was found after many trials that a blend of these earths in certain proportions gives a mantle that yields a pure white light, while any preponderance of one or another would impart some tint to the light. This proper blending of the constituents forms a great improvement on the first mantles, which generally shed a greenish light.

The mantles are made by an ingenious process, in which a network of cotton thread is knitted into the form of a tube; this is cut up into suitable lengths, and a piece attached to form the top. The network is then saturated with a solution of the nitrates of the rare earths above-mentioned, and dried on glass rods. After this a loop of asbestos thread is passed through the top, by which the mantle may be attached to its support. The mantle is now shaped to the required form, and the cotton thread burnt off, when a thin skeleton of the oxides is left reproducing the form of the original network. The mantle is again strongly heated, and after cooling is dipped into a solution of collodion, dried, and carefully laid in a box. The collodion serves to strengthen the mantle sufficiently for transit, for it is very frail, and would otherwise be liable to fall in pieces by slight shocks. Fig. 351_d_ is a full-sized representation of the completed mantle, and Fig. 351_e_ shows it mounted on the burner, where a rather small flame is allowed for the first time to play upon it, by which the collodion is quickly burnt off, and then the chimney-glass is placed over it, as in Fig. 351_a_. In the earlier forms of lamp the lighting of the gas was a matter requiring some delicacy of manipulation, for a rude shock, or an awkward touch might cause the mantle to crumble into ruin, but now the makers fit their lamps with a by-pass by which a very small flame is maintained within the lamp ready to light up the gas when that is fully turned on. (Fig. 351_c_.) The makers have also now made the lamps available for street lighting, to which the fragility of the mantle was formerly an obstacle, as it was liable to collapse by the tremor of the traffic. This risk has been obviated by providing a spring to support the mantle at the base. (Fig. 351_b_.)

The qualities of the Welsbach lamp have been examined by competent persons, and from the statements they supply, we extract the following particulars. The light is, for the same gas consumption, seven times that of an ordinary gas burner; more than four times that of an Argand burner; more than twice that of a “regenerative” lamp. It follows, of course, that, light for light, the products of combustion, such as carbonic acid, heat, &c., amount to only something like ⅙th of those produced by ordinary burners, and the consumption of the gas is perfect, there being absolutely no smoke. Though the mantles have to be renewed about three times a year, when the burners are in constant use, the total cost, light for light, is only ¼th of that of ordinary burners. The light of the Welsbach burner is whiter than ordinary gaslight. It is rich in the blue rays, and, therefore, more like daylight, permitting well the comparison of shades of colour, and it is excellently suited for workers with the microscope, &c. This new gas-lighting must also be a great boon to photographers using artificial illumination, for the actinic power is, with the same visual illumination, nearly twice that of the ordinary gas flame.

COAL-TAR COLOURS.

Coal-tar is an exceedingly complex material, being a mixture of a great number of different substances. The following table shows the chemical name of many of the substances obtainable from the coal-tar. It must not be supposed that these substances exist ready formed in the coal, and that they are merely expelled by the heat. We can understand better how heat, acting upon an apparently simple substance like coal, and one containing so few elements, is able to produce so large a variety of different bodies, if we remember that heat is the agent most often employed to effect chemical changes, and that from even two elements, variously combined, bodies differing entirely from each other are producible.

SUBSTANCES FOUND IN COAL-TAR.

a. COMPOUNDS OF CARBON AND HYDROGEN.

Hydrides of amyl, hexyl, heptyl, nonyl, and decyl. Amylene, hexylene, heptylene, octylene, nonylene, decylene; (paraffin). Benzol, toluol, xylol, cumol, cymol. Naphthalene. Anthracene. Pyrene. Chrysene.

b. COMPOUNDS OF CARBON, HYDROGEN, AND OXYGEN.

Phenol, cresol, phlorol. Rosolic acid, brunolic acid.

c. COMPOUNDS OF CARBON, HYDROGEN, AND NITROGEN.

Aniline, toluidine. Pyridine, picoline, lutidine, collidine, parvoline, coridine, rubidine, viridine. Leucoline, lepidine, cryptidine. Cespitine, pyrrol.

This list contains only the names of substances which have actually been found in the coal-tar, and it is certain that a number of products must have escaped notice. It is obvious, too, that by using coal of different kinds, and by varying the temperature and pressure at which the operation of distilling the coal is effected, we shall probably be able to increase the number of possible constituents of coal-tar almost indefinitely. The list above presents to the non-chemical reader a string of quite unfamiliar names; but, though the system of nomenclature in chemistry is far from perfect, yet each of these names has a meaning for the chemist beyond the mere designation of a substance. The chemical name aims at showing, or at least suggesting, the composition of a body and the general class to which it belongs. This may be illustrated by the names of hydro-carbons in the above list. The five compounds headed by benzol have many properties in common, and each one is entirely different in its chemical behaviour to those which follow amylene. The Greek numerals enter into the names of the latter, in order to express, in this case, the number of atoms of carbon which are supposed to be contained in each ultimate particle of the body. We write down in parallel columns the names of these two classes of bodies, together with the symbols which represent their composition, reminding the reader that the letter C represents carbon; the letter alone indicating one atom of that element, but, when followed by a small figure, it implies that number of carbon atoms; in like manner H, N, and O represent atoms of hydrogen, nitrogen, and oxygen respectively.

Hexylene C_{6}H_{12} Heptylene C_{7}H_{14} Octylene C_{8}H_{16} Nonylene C_{9}H_{18} Decylene C_{10}H_{20} Benzol C_{6}H_{6} Toluol C_{7}H_{8} Xylol C_{8}H_{10} Cumol C_{9}H_{12} Cymol C_{10}H_{14}

If these lists be carefully examined, it will be observed that there is a regular progression in the constituent atoms, so that each set of substances forms a series, the differences being always the same. The various bodies contained in the coal-tar are separated from each other by taking advantage of the fact that each substance has its own boiling-point; that is, there is a certain temperature, different for each body, at which it will rise into vapour quickly and continuously. Benzol, for example, boils at 82° C., toluol at 114° C., and phenol at 188° C.; so that, if we apply heat to a mixture of these three substances, the benzol will boil when the temperature reaches 82°, and will pass away in vapour, carrying off heat, so that the temperature will not rise until all the benzol has been driven off; then, when the temperature reaches 114°, the toluol will begin to come off, but not until that has all passed over into the receiver will the temperature rise above 114°; and the phenol remaining will distil only at 188°.

Another mode of separating bodies when mixed together is by treating them with a liquid which acts on, or dissolves out, some of the constituents, but not the rest. The coal-tar, as it is received from the gas-works, is placed in large stills, capable, perhaps, of holding several thousand gallons, and usually made of wrought iron. Stills sufficiently good for the purpose are commonly constructed from the worn-out boilers of steam engines. The application of heat, of course, causes the more volatile substances to come over first. These are condensed and collected apart until products begin to come off which are heavier than water. The first portion of the distillate, containing the lighter liquids, is termed “coal naphtha.” The process is continued, and heavier liquids come over, forming what is called in the trade the “dead oil.” Pitch remains behind in the retort, from which it is usually run out while hot, but sometimes the distillation is carried a step further.

The chief colour-producing substances contained in coal-tar are benzol, toluol, phenol, naphthalene, and anthracene. The aniline which is present in the tar is very small in amount, and if this ready-formed aniline were our only supply, it would be impossible to make colours from it on an industrial scale. The first of the above-named substances, benzol, was discovered by Faraday, in 1825, in liquid produced by strongly compressing gas obtained from oil. He called it bicarburet of hydrogen; but afterwards another chemist, having procured the same body by distilling benzoic acid with lime, termed it benzine. It readily dissolves fats and oils; and is used domestically for removing grease-spots, cleaning gloves, &c., and in the arts as a solvent of india-rubber and gutta-percha. It is a very limpid, colourless liquid, very volatile, and, when pure, is of a peculiar but not disagreeable odour. It boils at 82° C., and, cooled to the freezing-point of water, it solidifies into beautiful transparent crystals, a property which is sometimes taken advantage of to separate it in a state of purity from other liquids which do not so solidify.

Benzol is very inflammable, and its vapour produces an explosive mixture with air. The vapour, which is invisible, will run out of any leak in the apparatus, like water, and flow along the ground. Accidents have occurred from this cause, and a case is on record in which the vapour having crept along the floor of the works, was set on fire by a furnace forty feet away from the apparatus, the flame, of course, running back to the spot from which the vapour was issuing. Benzol is a dreadful substance for spreading fire should it become ignited, for, being lighter than water, it floats upon its surface, and therefore the flames cannot be extinguished in the ordinary way. The discovery of the presence of benzol in coal-tar was made by Hofman in 1845. It is obtained from the light oil of coal-tar by first purifying this liquid by alternately distilling it with steam and treating with sulphuric acid several times. The product so obtained is a colourless liquid, sold as “rectified coal naphtha,” which, however, has again to be several times re-distilled with a careful regulation of the temperature, so that the benzol may be distilled off from other substances, boiling at a somewhat higher temperature, with which it is mixed. Even then the resulting liquid (commercial benzol) contains notable quantities of toluol. If benzol be added in small quantities at a time to very strong and warm nitric acid, a brisk action takes place, and when after some time water is added, a yellow oily-looking liquid falls to the bottom of the vessel. The benzol will have disappeared, for nitric acid under such circumstances acts upon it by taking out of each particle an atom of hydrogen, which it replaces by a group of atoms of nitrogen and oxygen, and, instead of benzol, we have the yellow oil, nitro-benzol. Chemists are accustomed to represent actions of this kind by what is called a chemical equation, the left-hand side showing the symbols representing the constitution of the bodies which are placed together, and the right hand the symbols of the bodies which result from the chemical action. Here is the equation representing the action we have described:

C_{6}H_{6} + NO_{2}OH = C_{6}H_{5}(NO_{2}) + HOH Benzol. Nitric acid. Nitro-benzol. Water.

Nitro-benzol has a sweet taste and a fragrant odour. It is known in commerce under the names of artificial oil of bitter almonds and essence of mirbane, and it has been used for perfuming soap. The chemical action between benzol and concentrated nitric acid is so violent that, when nitro-benzol first had to be manufactured on the large scale, great difficulty was experienced on account of the serious explosions which occurred. The apparatus now used in making nitro-benzol on the large scale is represented in Fig. 353, which shows some of the cast-iron pots, of which there is usually a long row. These pots are about 4½ ft. in diameter, and the same in depth. Each is provided with a stirrer, which is made to revolve by a bevil-wheel, c, on its spindle, working with a pinion on a shaft, b, driven by a steam engine. A layer of water is kept on the tops of the lids, the water being constantly passed in and drawn off through the pipes, d, in order to keep it cool. For the chemical action is, as usual, attended with heat, which vaporizes some of the benzol, but the cold lid re-condenses the vapour, which would otherwise escape with the nitrous fumes that pass off by the pipe, a. There is at e an opening, through which the material may be introduced, and in the bottom of the vessel is an aperture through which the products may be drawn off. Fig. 354 shows a section of one of the cast-iron vessels, and exhibits the mode in which the spindle of the stirrer passes through the lid. In the cup, a, filled with a liquid, a kind of inverted cup, which is attached to the spindle, turns round freely. It would not do to choose water for the liquid in this cup, for water would, by absorbing the nitrous fumes, form an acid capable of attacking and destroying the spindle. Nothing has been found to answer better for this purpose than nitro-benzol itself. The charge introduced into these vessels is a mixture of nitric and sulphuric acids together with the benzol. During the action, which may last twelve or fourteen days, no heat is applied, for the mixture becomes hot spontaneously, and in fact care must be taken that it does not become too hot. The nitro-benzol thus obtained is purified by washing with water and solution of soda.

If nitro-benzol were brought into contact with ordinary hydrogen gas, no action whatever would take place. But it is well known to chemists that gases which are just being liberated from a compound have at the instant of their generation much more powerful chemical properties than they possess afterwards. Gases in this condition are said to be in the nascent state. If we submit nitro-benzol to the action of nascent hydrogen we find a remarkable change is produced. This change consists, first, in the hydrogen robbing the nitro-benzol of all its oxygen atoms; second, in the addition of hydrogen to the remainder; third, in some re-arrangement of the atoms, by which a new body is formed. Not that these changes are successive, or that we actually know the movement of atoms, but we are thus able to form ideas which correspond with the final result. The new substance is named aniline. It is regarded by chemists as a base; that is, a substance capable of neutralizing and combining with an acid to form a salt. Its composition is represented by the symbols C_{6}H_{5} H_{2}N. Aniline was found in coal-tar in 1834, and even its colour-producing power was noticed, for its discoverer named it kyanol, in allusion to the blue colour it produced with chloride of lime. Later it was obtained by distilling indigo with potash, and hence received its present name from anil, the Portuguese for indigo. The quantity of aniline contained in the tar is quite insignificant.

Aniline is prepared from nitro-benzol on the large scale by heating it with acetic acid and iron filings or iron borings, a process which rapidly changes the nitro-benzol into aniline. The equation representing the change is—

C_{6}H_{5}NO_{2} + H_{6} = C_{6}H_{5}H_{2}N + 2H_{2}O. Nitro-benzol. Hydrogen. Aniline. Water.

The operation is effected in the apparatus represented in Fig. 355. It consists of a large iron cylinder, within which works a paddle on a vertical revolving spindle, which, being hollow, is also a pipe to convey high pressure steam within the apparatus. Fig. 356 is a section of the hollow spindle, in which f is the pivot at the bottom of the cylinder on which it turns; d is the stirring paddle; e is an aperture admitting the steam from the pipe, c, forming the shaft of the paddle, which is made to revolve by the bevil-wheel. The steams enters by the elbow-pipe, which has a nozzle ground to fit the head of the vertical revolving pipe, upon which it is pressed down by the screw. When the materials have been introduced into the cylinder, the stirrer is set in motion, and superheated steam is sent down the pipe; the aniline is volatilized and passes with the steam through the pipe, which is connected with a worm surrounded by cold water. The aniline is purified by another distillation over lime or soda. When pure, aniline is a colourless, somewhat oily-looking liquid, of a feeble aromatic odour. Under the influence of light and air it becomes of a brownish tint, in which condition it usually presents itself in commerce. It scarcely dissolves in water, but is readily soluble in alcohol, ether, &c.

It was Mr. Perkin who, in 1856, first obtained from aniline a substance practically available for dyeing. Let it be noticed that when Mr. Perkin discovered aniline purple, he was not engaged in searching for dye-stuffs, but was carrying on a purely scientific investigation as to the possibility of artificially preparing quinine. With this view, having selected a substance into the composition of which nitrogen, hydrogen, and carbon enter in exactly the same proportions as they occur in quinine, but differing from it by containing no oxygen, he thought it not improbable that by oxidizing this body he might obtain quinine. In this he was disappointed, for the result was a dirty reddish-brown powder. Being desirous, however, of understanding more fully the nature of this reddish powder, he proceeded to try the effects of oxidation on other similarly constituted but more simple bodies. For this purpose he fortunately selected aniline, which, when treated with sulphuric acid and bichromate of potash, he found to yield a perfectly black product. Persevering in his experiments by examining this black substance, he obtained, by digesting it with spirits of wine, the now well-known “aniline purple.” Mr. Perkin, having determined to make the aniline purple on the large scale, patented his process, and succeeded in overcoming the many obstacles incident to the establishment of a new manufacture requiring as its raw material products not at that time met with as commercial articles. The process is now carried on the large scale by mixing sulphuric acid and aniline in the proportions in which they combine to form the sulphate of aniline, and dissolving by boiling with water in a large vat. Bichromate of potash is dissolved in water in another large vat. When both solutions are cold, they are mixed together in a still larger vessel and allowed to stand a day or two. A fine black powder settles on the bottom of the vessel in large quantities; this is collected in filters, washed with water, and dried. This powder is not aniline purple alone, but a mixture of this with other products, presenting a very unpromising appearance; but when it has been digested for some time with diluted methylated spirit, all the colouring matter is dissolved out, and is obtained from the solution by placing the latter in a still, where the spirit is distilled off and collected for future use, while all the colouring matter remains behind, held in solution by the water. From this aqueous solution the mauve is thrown down by adding caustic soda. It is collected, washed, and drained until of a pasty consistence, in which condition it is sent into the market. It can be obtained in crystals, but the commercial article is seldom required in this form, as the additional expense is not compensated by any superiority in the practical applications of the colour. Mauve is readily soluble in spirits of wine, but not very soluble in water. Its tinctorial power is so great that one-tenth of a grain suffices to impart quite a deep colour to a gallon of water. Silk and woollen fabrics have an extraordinary attraction for this colouring matter, which attaches itself very firmly to their fibres. If some white wool is dipped into even a very dilute solution, the colour is quickly absorbed. Mauve is more permanent than any other coal-tar colour, being little affected by the prolonged action of light.

Mauve is chemically a salt of a base which has been termed “mauveine.” Mauveine itself is a nearly black crystalline powder, which forms solutions of a dull blue-violet tint, but when an acid is added to such a solution the tint is at once changed to purple. Mauveine is a powerful base, displacing ammonia from its compounds. The commercial crystallized mauve is the acetate of mauveine.

The process by which Mr. Perkin originally obtained mauve from aniline evidently depends upon the well-known oxidizing property of bichromate of potash, and experiments were accordingly made with other, oxidizing bodies and aniline; in fact, patents were taken out for the use of nearly every known oxidizing chemical. Three years after Mr. Perkin’s discovery of mauve, M. Verguin, of Lyons, obtained, by treating crude aniline with chloride of tin, the bright red colouring matter now known as magenta. It was found also that crude aniline, when treated with other metallic chlorides, nitrates, or other salts, which are oxidizing agents less powerful than bichromate of potash, yields this bright red colouring matter. A process patented by Medlock, in 1860, in which arsenic acid is the oxidizing agent, has almost entirely superseded, in England at least, all the others yet proposed for the manufacture of magenta. It is not a little remarkable that magenta would not have been discovered had M. Verguin and others operated on pure aniline instead of on the ordinary commercial article. For it was found subsequently by Dr. Hofman that pure aniline cannot be made to yield magenta: the presence of another body is necessary. A reference to the table of coal-tar constituents will show that there is a hydro-carbon named “toluol.” This substance is of a similar nature to benzol, and has a boiling-point so little above that of benzol, that in the rough methods of separation usually employed, a notable quantity of toluol is carried over with the benzol, and is always present in the commercial article. In the processes which benzol undergoes for conversion into aniline, the toluol accompanies it in a series of parallel transformations, resulting in the production of a base termed “toluidine”—similar to aniline—being, however, in its pure state a solid at ordinary temperatures. We write down the symbols representing the composition of the bodies formed in the two cases in order to clearly show this:

Benzol C_{6}H_{6} Nitro-benzol C_{6}H_{5}(NO_{2}) Aniline C_{6}H_{5}NH_{2} Toluol C_{7}H_{8} Nitro-toluol C_{7}H_{7}(NO_{2}) Toluidine C_{7}H_{7}NH_{2}

This aniline prepared from commercial benzol always contains some toluidine; and it is essential for the production of magenta that this substance should be operated on along with the aniline. Whether the presence of some toluidine is also necessary for the production of mauve and other colours is not yet known, but they are always prepared from commercial benzol. It is certain that pure aniline yields no magenta, neither does pure toluidine; but a mixture supplies it in abundance. For the preparation of magenta the best proportions for this mixture would be about three parts of aniline to one of toluidine; but, in practice, it is not necessary to obtain the two substances separately, as benzol, mixed with a sufficient quantity of toluol, may be obtained by regulating the distillation. The apparatus used in the production of magenta is shown in Fig. 352. It consists of a large iron pot set over a furnace in brickwork, and having a lid with a stuffing-box, through which passes a spindle carrying a stirrer. A bent tube rises from the lid, and is connected with a worm surrounded by cold water, for the purpose of condensing the aniline which is vapourized in the process. The aniline, containing a due amount of toluidine, is mixed in this apparatus with about one and a half times its weight of a saturated solution of arsenic acid (H_{3}AsO_{4}). The fire is lighted and kept up for several hours: water first, and lastly aniline, distil over. When the operation is ended, steam is blown through the apparatus, thus carrying off an additional portion of aniline. The crude product is then boiled with water, the solution filtered, and common salt added, which precipitates an impure magenta. This is afterwards dissolved and recrystallized several times. The crystals of this magenta—like those of many of the coal-colour products—have a peculiar greenish metallic lustre; they dissolve in warm water, forming a deep purplish-red solution. The chemical composition of magenta has been investigated by Dr. Hofman, who found it to be a salt of an organic base, to which he gave the name of “rosaniline.” This rosaniline is easily obtained from magenta by addition to its solution of an alkali. While all its salts are intensely coloured, rosaniline itself is a perfectly colourless substance, becoming reddened by exposure to the air, as it absorbs carbonic acid, thus passing to the condition of a salt. Rosaniline, then, displays its chromatic powers only when it is combined with an acid. This property is sometimes shown at lectures in a striking manner by dipping a piece of paper into a colourless solution of rosaniline, and exposing it to the air, when, as the rosaniline absorbs carbonic acid, the paper changes from white to red. A more elegant form of the same experiment is to dip a white rose into a solution of rosaniline containing a little ammonia. As the ammonia escapes, or is expelled by a current of warm air, the same kind of action occurs, and the white rose changes to red—as if by magic, the emblem of the House of York is transformed into the badge of Lancaster! The chemical nature of rosaniline is regarded as analogous to that of ammonia—it is, in fact, looked upon by chemists as a sort of ammonia, in each particle of which some atoms of hydrogen have been replaced by certain groups of carbon and hydrogen atoms—some of these groups being derived from the aniline and others from the toluidine. The particular salt of rosaniline which constitutes the crude product of the action on the aniline and toluidine, depends on the substance employed to effect the oxidation. If a chloride, the resulting product is chloride of rosaniline; if a nitrate, it is the nitrate; and so on. The magenta which is formed in the first instance by the process we have described is an arseniate of rosaniline; but in the subsequent processes, it is converted into the chloride—the salt usually sold as magenta. Other salts of rosaniline are made on the large scale—especially the acetate, the beautiful crystals of which have the advantage of being very soluble.

Magenta attaches itself strongly to animal fibres, but the colour is somewhat fugacious under the action of sunlight. It is used not only as a dye, but more largely as the raw material from which a number of other beautiful colours are obtained. For this reason it is manufactured on an enormous scale, thousands of tons being produced annually, and the money value of the colour produced from it must be reckoned by thousands of pounds. Yet aniline was a few years ago merely a curiosity never met with out of the laboratory of the scientific chemist. It is stated that a single firm now makes more than twelve tons of aniline weekly, and on its premises may be seen tanks, in each of which 30,000 gallons of magenta solution is depositing its crystals. If a salt of rosaniline be heated with aniline, the colour changes gradually through purple to blue, while ammonia is at the same time given off. This is the colour known as aniline blue, “bleu du Lyons,” &c. In its preparation it has been found that the best results are obtained by employing the salt of some weak acid—acetate of rosaniline, for example—and pure aniline, that is, aniline free from toluidine. The operation is conducted in iron pots very similar to those used in making magenta, but smaller. These pots are not set over a fire, but a number of them are placed in a large vessel containing oil, by which they can be maintained at a regulated temperature when the oil is heated. The crude product undergoes several purifications, and the aniline blue is supplied in commerce in powder, or dissolved in spirits of wine. It is insoluble in water, and this has been an obstacle to its employment; but recently a similar substance has been obtained in a soluble form, and is extensively used for dyeing wool, under the name of “Nicholson’s blue.” Other blues have been similarly prepared, and from the same two substances, magenta and aniline, a colour known as “violet imperial” was formerly made in very large quantities, but it has been superseded by the colours about to be described. It may be well to mention that these blues and violets have been found to contain bases formed of rosaniline, in which one, two, or three atoms of hydrogen are replaced by the group C_{6}H_{5}. This group of atoms will be noticed to belong to aniline, and chemists have named it phenyl, and, therefore, bases of these coloured salts are respectively named phenyl-rosaniline, di-phenyl-rosaniline, tri-phenyl-rosaniline. But Dr. Hofman found that other groups of atoms besides C_{6}H_{5} may be made to take the place of H in rosaniline. By acting on rosaniline or its salts with iodides of ethyl, C_{2}H_{5}I, or iodide of methyl, CH_{3}I, he obtained a beautiful series of violets, of which many shades could be produced, varying from red-purple to blue. These are the colours so well known as Hofman’s violets, and are prepared on the large scale by heating a solution of magenta (chloride of rosaniline) in alcohol or wood spirit, with the iodide of ethyl or the iodide of methyl. The nature and proportions of the ingredients are regulated according to the tint required. The vessels are hermetically closed during the heating, which is accomplished by steam admitted into a steam-jacket surrounding the vessel. The crude product has to be separated from the substances with which it is mixed, and the colouring matter is finally obtained, presenting in the solid state the peculiar semi-metallic lustre so characteristic of these products. Like the other colours, Hofman’s violets are salts of colourless bases, which, as indicated above, are substitution products of rosaniline. The tints they produce incline to red, violet, or blue, according as one, two, or three hydrogen atoms are replaced by the ethyl or methyl groups. Colours have also been obtained from mauve and iodide of ethyl—for example, the dye known in commerce as “dahlia.” Other colours are procured from magenta by treating it with various compounds: one such is the “Britannia violet,” discovered also by Mr. Perkin, who procures it from magenta and a hydrocarbon-bromide derived from the action of bromine or common turpentine. This is a very useful colour, and is largely used in dyeing and printing violets, of which any shades may be obtained.

Another derivative of rosaniline is the aniline green. It is obtained by dissolving the rosaniline salt in dilute sulphuric acid, adding crude aldehyde (a substance obtained by acting with oxidizing agents on alcohol). The mixture is heated until a sample dissolves in acidulated water with a blue tint; it is poured out into boiling water containing in solution hyposulphite of sodium, boiled, the liquid filtered; and the green dye, if required in the solid state, is precipitated by carbonate of sodium. Aniline green dyes wool and silk, the latter especially, of a magnificent green; perhaps as beautiful a colour as any of the coal-tar series, and one which has the singular advantage among greens of looking as beautiful in artificial light as in daylight. The manner in which this dye was discovered is somewhat curious. It is related by Mr. Perkin of a dyer, named Chirpin, that he was trying to render permanent a blue colouring matter, which had been found could be produced from rosaniline by the action of aldehyde and sulphuric acid. After a number of fruitless attempts at fixing it, he confided his perplexities to a photographic friend, who evidently thought that if it was possible to fix a photograph, anything else might be fixed in like manner, for he recommended his confidant to try hyposulphite of sodium. On making the experiment, however, the dyer did not succeed in fixing his blue, but converted it into the splendid aldehyde green. Like other colouring matters we have described, this is a salt of a colourless base containing sulphur. Like rosaniline, the colourless base takes on the characteristic colour of its salts by merely absorbing carbonic acid from the air.

Again, by a modification of the process for producing the Hofman violets, another green of an entirely different constitution may be obtained. It is bluer in tint than the former, and is much used for cotton and silks, under the name of “iodine green.”

In the manufacture of magenta there is formed a residuum or bye-product, consisting of a resinous, feebly basic substance, from which Nicholson obtained a dye, imparting to silk and wool a gorgeous golden yellow colour. This dye cannot be obtained directly, but is always produced in greater or less quantity when magenta is made on the large scale, and is separated during the purification. By first dyeing the silk or wool with magenta, and then with this dye, which is commercially known as “phosphine,” brilliant scarlet tints are obtained. The yellow colours have been found to be salts of a base termed chrysaniline, a sort of chemical relative of rosaniline, as may be seen in comparing the formulæ which represent their constitution, with which we place also the symbol for another substance obtained by submitting rosaniline to the influence of nascent hydrogen. This body, leucaniline, again yields rosaniline very readily when the hydrogen is removed by oxidizing agents. It will be noticed that the three bodies form a series the members of which differ only by H_{2}, thus indicating their close relationship.

C_{20}H_{17}N_{3} Chrysaniline. C_{20}H_{19}N_{3} Rosaniline. C_{20}H_{21}N_{3} Leucaniline.

Some idea will have been obtained from the foregoing particulars of the great colour-supplying capabilities of aniline; but we have not yet exhausted the utility of this interesting substance. It is probable that the letters on the page now under the reader’s eye owe their blackness to an aniline product. For after all the salts furnishing the lovely tints we have mentioned have been extracted, there is in their manufacture a final residuum, and from this an intense black is obtained, which is largely used in the manufacture of printing-ink.

We have mentioned phenol as a substance yielding colours. Phenol is the body now so well known as a disinfectant under the name of “carbolic acid,” a name given to it by its discoverer, Runge, who prepared it from coal-tar, in 1834. Phenol forms colourless crystals, which dissolve to some extent in water, and very readily in alcohol. It is a powerful antiseptic, that is, it arrests the process of putrefaction in animal or vegetable bodies, and it is also highly poisonous. The constitution of phenol is given by the formula C_{6}H_{5} OH, in which the reader will recognize the same group of atoms already indicated as entering into the aniline derivatives. From some of these phenol may in fact be obtained, and although it cannot be formed directly from benzol, phenol can be made to furnish benzol. When crude phenol is treated with a sulphuric acid and oxalic acid, a substance is obtained which presents itself as a brittle resinous mass of a brown colour, with greenish metallic lustre. This substance is called rosolic acid by chemists, but in commerce it is known as aurine, and is used for dyeing silk of an orange colour, which, however, is not very permanent. But by heating rosolic acid with liquid ammonia, a permanent red dye is procured which has been termed peonine, and has been much used for woollen goods. But it lately had the reputation of exerting a poisonous action, producing blistering and sores when stockings or other articles dyed with it were worn in contact with the skin. It is now, therefore, less extensively employed. Coralline, another body identical with or very similar to the former, is similarly prepared from rosolic acid by heating it with ammonia under pressure.

Again, by heating coralline with aniline, a blue dye, known as “azurine,” or “azuline,” was formerly made in large quantities; but it has been supplanted by the aniline blues already described.

When phenol is acted upon by nitric acid new compounds are produced, standing in the same relation to phenol as nitro-benzol does to benzol. The final result of the action of nitric acid on phenol is picric acid, called also “carbazotic acid,” and, more systematically, “tri-nitro-phenol;” for it is regarded as phenol in which three of the hydrogen atoms have been replaced by the group NO_{2} thus, C_{6}H_{2}(NO_{2})_{3} OH. It forms bright yellow-coloured crystals, and its solution readily imparts a bright pure yellow colour to wool, silk, &c. It received the name of picric acid (πικρος, bitter) from the exceedingly bitter taste of even an extremely diluted solution. It is said that picric acid is employed as an adulterant in bitter ale instead of hops. Now, the colouring power of picric acid is so great, that even the minute quantity which could be used to impart bitterness to beer is recognizable by dipping a piece of white wool into the beer, when, if picric acid be present, the wool acquires a clear yellow tint. Besides its employment as a yellow, it is useful for procuring green tints by combination with the blues. Picric acid again furnishes, by treatment with cyanide of potassium, a deep red colour, consisting of an acid which, when combined with ammonia, furnishes a magnificent colouring material—which is, in fact, murexide, a dye identical with the famous Tyrian purple of the ancients, and formerly obtainable only from certain kinds of shell-fish.

Naphthaline—another of the colour-yielding substances of coal-tar—is, like benzol, a hydro-carbon, but one belonging to quite another chemical series. Its formula is C_{10}H_{8}, and it has an interest to chemists altogether apart from its industrial uses, from having been the subject of the classic researches of the French chemist, Laurent—researches which resulted in the introduction of new and fertile ideas into chemical science, contributing largely to its rapid progress. Naphthaline forms colourless crystals, which, like camphor, slowly volatilize at ordinary temperatures, and are readily distilled in a current of steam. It is thus sufficiently volatile to escape complete deposition in the condensers of the gas-works, and to be partly carried over into the mains, where its collection occasions some trouble. Nitric acid acts upon naphthaline in a manner analogous to that in which it acts on benzol, forming nitro-naphthaline, which, in its turn, submitted to the action of iron filings and acetic acid, is transformed into a base called “naphthylamine.” The salts of naphthylamine are coloured products which, in some cases, have been found available as dyes. There is a crimson colour, and a yellow largely used under the name of “Manchester yellow,” for imparting to silk and wool a gorgeous golden yellow colour. Another coloured derivative of naphthaline, called “carminaphtha,” was discovered by Laurent in the course of his researches.

It would be easy to fill this volume with descriptions of the properties, and modes of preparing the numerous colouring matters that have been obtained from coal-tar products. In order to give the reader an idea of the extent to which the tar products have been made to minister to our sense of the beautiful, a list is here given of the principal colouring matters from these sources that have been employed in the arts. The various names under which a product has been commercially known are in most cases given. It must be understood that the same name is frequently applied to products chemically distinct, and some of the names which appear as synonyms may also in reality indicate different substances.

LIST OF COAL-TAR COLOURS.

I. COLOURS DERIVED FROM ANILINE AND TOLUIDINE.

Blues and Violets.

Mauve, aniline purple, Perkin’s violet, violine, mauve, rosaniline, anodising, &c.

Aniline blue, rosaniline blue, Hofman’s blue, bleu de Paris, bleu de Lyons, bleu de Mulhouse, bleu de Mexique, bleu de nuit, bleu lumière, night blue.

Hofman’s blue. Nicholson’s blue, soluble blue. Hofman’s violet, rosaniline violet.

A long series of red and blue violets, bearing Hofman’s name and distinguished in commerce by adding R or B, according to the redness or the blueness of the tint, ranging from RRRR to BBBB.

Dahlia. Toluidine blue. Violet de Paris. Mauvaniline. Violaniline. Regina blue, opal blue, bleu de Fayolle, violet de Mulhouse. Britannia violet. Violet imperial. And many others.

Reds.

Aniline red, new red, magenta, solferino, aniline, rougé, roseine, azaline. Rubine, rubine imperial. Chrysaniline red. (The above are all salts of rosaniline) Xylidine, tar red, soluble red.

Yellows.

Chrysaniline, phosphine, aniline yellow, yellow fuschine. Chrysotoluidine. Dinaline. Field’s orange.

Greens.

Aldehyde green, aniline green, viridine, emeraldine. Iodine green, iodide of methyl green, iodide of ethyl green. Perkin’s green.

Browns.

Havanna brown. Bismarck brown, aniline brown, Napoleon brown, aniline maroon.

Greys and Blacks.

Aniline grey, argentine. Argentine black.

II.—COLOURS DERIVED FROM PHENOL.

Blues and Violets.

Isopurpuric acid, Grénat. Azuline, azurine.

Reds.

Picramic acid. Coralline, peonine. Red coralline.

Yellows.

Picric acid, carbazotic acid. Aurine, rosolic acid.

Green.

Chloropicrin.

Browns.

Picrate of ammonia. Isopurpurate of potash. Phenyl brown, phenicine.

III.—COLOURS DERIVED FROM NAPHTHALENE.

Reds.

Pseudoalizarine, naphthalic red. Roseonaphthaline, carminaphtha.

Yellows.

Binitronaphthaline, naphthaline yellow, golden yellow, Manchester yellow. And others.

The introduction of aniline colours into dyeing and calico-printing has caused quite a revolution in these arts, the processes having become much more simple, and the facilities for obtaining every variety of tint largely increased. The arts of lithography, type-printing, paper-staining, &c., have also profited by the coal-tar colours. For such purposes the colour is prepared by fixing it on alumina, a process in which much difficulty was at first experienced, for the colours are themselves almost all of a basic nature. The desired result is now attained by fixing them on the alumina with tannic or benzoic acid. These lakes produce brilliant printing-inks, which are extensively used. The aniline colours are also employed for coloured writing-inks, tinted soaps, imitations of bronzed surfaces, and for a variety of other purposes.

Not many years ago coal-tar was a valueless substance: it was actually given away by gas-makers to any one who chose to fetch it from the works. It was then “matter in the wrong place;” but Mr. Perkin’s discovery led to its being put in the right place, and it has become the raw material of a manufacture creating an absolutely new industry, which has developed with amazing rapidity. This industry dates from only 1856, and in 1862 the annual value of its products was more than £400,000. Dr. Hofman, in reporting on the coal-tar colours shown at the Paris Exhibition of 1867, computed the value at that time at about £1,250,000, although the products were much cheaper than before. Large manufactories have been established in Great Britain, in France, Germany, Switzerland, America, and other countries. The possibility of such an industry is an interesting illustration of the manner in which the progress made in any one branch of practical science may lead to unexpected developments in other quarters. The quantity of aniline obtained from coal-tar is very small compared to the amount of coal used, as may be seen from the following table, in which the respective weights of the various products required in the manufacture of mauve are arranged as given by Mr. Perkin for the produce of 100 lbs. of coal.

lbs. oz. Coal 100 0 Coal-tar 10 12 Coal-tar naphtha 0 8½ Benzol 0 2¾ Nitro-benzol 0 4¼ Aniline 0 2¼ Mauve 0 0¼

From this we may perceive that had not the manufacture of gas been greatly extended, so as to yield a large aggregate produce of tar, the requisite supply for the manufacture of aniline would not have been attainable; and the industrial application of the previously worthless bye-product reacts upon gas manufacture by cheapening the price of that commodity, thus tending still more to extend its use.

Although anthracene has already been named as one of the colour-producing substances found in coal-tar, we have not in the list of coal-tar colours included the colouring matter which anthracene is capable of yielding. The reason is that this case stands apart in some respects from the rest. The colours derived from aniline and the other substances already enumerated are instances of the production of bodies not found in nature—mauve, magenta, &c., do not, so far as we know, exist in nature. Their artificial formation was a production of substances absolutely new. The colour of which we have now to treat is, on the other hand, found in nature, and from its occurrence in the rubia tinctoria, the roots of that plant have for ages been employed as a source of colour, and are well known in this country as “madder.” The plant is grown largely in Holland, in France, in the Levant, and in the south of Russia.[18] Madder is used in enormous quantities for dyeing reds and purples: the well-known “Turkey red” is due to the colouring matter of this root. The total annual value of the madder grown is calculated to reach nearly 2½ million pounds sterling. More than forty years ago it was discovered that the madder-root yielded a colouring substance, to which the name of “alizarine” was bestowed, from alizari, the commercial designation of madder in the Levant. The alizarine does not exist in the fresh root, but is produced in the ordinary processes of preparing the root and dyeing with it, in consequence of a peculiar decomposition or fermentation. Alizarine may be procured from dried madder by simply submitting it to sublimation, when beautiful orange needle-shaped crystals of alizarine may be obtained. It is nearly insoluble in water, but readily dissolves in hot spirits of wine. Acids do not dissolve it, but potash dissolves it freely, striking a beautiful colour; with lime, barytes, and oxide of iron, it forms purple lake, and with alumina a beautiful red lake. According to Dr. Schunck, of Manchester, to whose investigations we are indebted for much of our knowledge of madder, the root contains a bitter uncrystallizable substance called “rubian,” which, under the action of certain ferments, and of acids and alkalies, is decomposed into a kind of sugar, and into alizarine and other colouring matters. The ferment, which in the process of extracting the colouring matter from the roots causes the formation of alizarine, is contained in the root itself.

Footnote 18:

The natural Order to which the madder plant belongs is interesting from the number of its members which supply us with useful products. That valuable medicine, quinine, is obtained from plants belonging to this family, as is also ipecacuanha, and other articles of the materia medica. Coffea arabica, which furnishes the coffee-berry, is another member.

We have already seen how an investigation relating to a question of pure chemical science accidentally led Mr. Perkin to the discovery of mauve—the precursor of the long range of beautiful colours already described. The mode of artificially preparing alizarine, so far from being an accidental discovery, was sought for and found in 1869 by two German chemists, Graebe and Liebermann. The researches of these chemists were conducted in a highly scientific spirit. Instead of making attempts to produce alizarine by trying various processes on first one body, then another, to see if they could hit upon some tar product, or other substance, which would yield the desired product, they began by operating analytically on alizarine itself. Just as a mechanic ignorant of horology, required to make a watch, would be more likely quickly to succeed in his task by taking a watch to pieces to see how it is put together, than if he had tried all manner of arranging springs and wheels until he hit upon the right way; so these chemists set themselves to take alizarine to pieces, in order to see from what materials they might be able to put it together. They decomposed alizarine, and among the products found a hydro-carbon identical in all its properties with anthracene.

Anthracene was discovered in coal-tar by Laurent in 1832, and its properties were investigated by Anderson in 1862. It may be remarked that such investigations were not conducted with a view to any industrial uses of anthracene, but merely for the sake of chemistry as a science. Certainly no one could have supposed at that time that the slightest relation existed between anthracene and madder. Anthracene is a white solid hydro-carbon, which comes over only in the last stages of the distillation of coal-tar, accompanied by naphthaline, from which it is easily separated by means of spirits of wine, by which the naphthaline is readily dissolved, but the anthracene scarcely. Anderson, in 1861, discovered, among other results, that anthracene, C_{14}H_{10}, by treatment with nitric acid became changed into oxy-anthracene, C_{14}H_{8}O_{2}; and this reaction we shall see is a step in the process of procuring alizarine from anthracene. Phenol, as already mentioned, can be made to yield benzol, by a process of deoxidization. With a view to similarly obtaining a hydro-carbon from alizarine, Graebe and Liebermann passed its vapours over heated zinc filings, and thus produced anthracene from alizarine. It now remained to find a means of reversing this process, that is, so to act on anthracene as to produce alizarine, and this was effected by treating anthracene with bromine, forming a substance which, on fusing with caustic potash, yielded alizarate of potash, from which pure alizarine resulted by treatment with hydrochloric acid. A much cheaper method was, however, necessary for manufacturing purposes, and it was found in a process by which oxy-anthracene, C_{14}O_{8}H_{2}, is treated at a high temperature with strong sulphuric acid, and the product so formed heated with a strong solution of potash, yielding alizarate of potassium as before. Many other interesting substances appear to be formed in the reactions, but the nature of these bodies has as yet been imperfectly investigated. No doubt whatever can be entertained of the identity of natural with artificial alizarine; and the production of this substance, the first instance of a natural colouring matter made artificially, may be regarded as a great triumph of chemical science. It was not long ago supposed that the chemical bodies found in plants or animals, or produced by vital actions, could not possibly be formed by any artificial process from their elements. The laws which presided at their formation were, it was conceived, wholly different from those which governed the chemicals of the laboratory, for they were held to act exclusively under the influence of a mysterious agent, namely, “vital force.” It was supposed, for example, that from pure carbon, oxygen, and hydrogen, no chemist would ever be able to produce such a compound as acetic acid. Accordingly the domain of chemical science, previous to the end of the first quarter of the present century, was divided by an impassable barrier into the two regions of organic and inorganic chemistry. Now, however, the chemist is able to build up in his laboratory from their very elements a great number of the so-called organic bodies. And it is quite possible to do this in the case of alizarine; that is, a chemist having in his laboratory the elements, hydrogen, carbon, oxygen, &c., could actually build up the substance which gives its value to madder.

The quantity of anthracene procurable from coal-tar is, unfortunately, comparatively small, for it is found that from the distillation of 2,000 tons of coal only one ton of anthracene can be obtained. The use of artificial alizarine would doubtless entirely supplant the employment of madder-root if anthracene could be obtained in larger quantities; and the change would be highly advantageous to this country, for as no madder is grown in Great Britain, and we consume nearly half the whole annual growth, it follows that every year a million pounds sterling go out of the country for this commodity. When anthracene is produced from coal in sufficient abundance, this sum will be available for the support of our own population. In the meantime, the manufacture of artificial alizarine is restricted only by the supply of its raw material.

The foregoing paragraphs of the present article, which were written for the first edition of this work, not long after the introduction of artificial alizarine, require some supplementary reference to the subsequent progress of discovery and to the increased importance of the manufacture of the coal-tar colours on the large scale. Since the first introduction of alizarine as a commercial product, the substance has received much attention from chemists. The constitution of the body called above oxy-anthracene is now better understood, and its chemical relationship is more clearly indicated by the systematic name of anthraquinone, which it now bears. The process of the manufacture of alizarine has received some advantageous modifications, and the artificial product may now be said to have entirely displaced the madder-root in dyeing. But, more than this, chemists have found means of preparing a number of “derivatives” of alizarine, many of which are either colouring matters or are easily converted into such. Nearly thirty of these substances have been described, and several of them have found extensive industrial applications. We may mention alizarine blue, C_{17} H_{9} NO_{4}, and another substance, produced by combining that with sodium bi-sulphite, and having the formula C_{17} H_{9} NO_{4} 2Na H SO_{3}. This last, manufactured largely, and sold under the name of “alizarine blue S.,” is remarkable for being one of the most permanent of all colouring matters. It is said to be a faster colour than even indigo blue, which, indeed, it is rapidly replacing in dyeing, where it is applicable to cotton with a chromium mordant and to silk with one of alumina. Two other colouring matters have also been derived from anthracene, and are much used in dyeing; one is commercially named anthracene purple, the other is anthracene green, which supplies the calico printer with very fast shades of olive-green.

Several of the substances enumerated in the list of coal-tar colours, in pages 689 and 690, are now but little used, or altogether abandoned in dyeing and calico printing, because either their beautiful hues prove too fugitive, or other bodies of the same class can be produced at a much cheaper rate. The range of choice is now of the amplest, for chemical discovery has been wonderfully active, but in many cases the real nature and relationship of the artificial colouring matters enumerated above have only quite recently been made out. Mauve (now called rosaline), for example, the oldest of all the colour-tar colours, and one which, as we have seen, was manufactured on an extensive scale many years ago, is now scarcely made at all, because much cheaper violets have taken its place. The science of the tinctorial substances has lately taken a much more distinct form, and this knowledge has borne fruit for industrial purposes. It would be out of place here to review what has been done in this way, but a few facts will show the richness of the field. It was only in 1886 that the true chemical constitution of a class of coal-tar derivatives, called azines, was first made out. They present themselves as pale yellow or orange coloured crystallized solids, which melt at a comparatively high temperature and may be distilled without decomposition. Although highly coloured substances themselves, before they are converted into fast dyes they require further treatment, which introduces into their molecules another group of atoms. An almost indefinite number of such compounds are theoretically possible, but from only a very few of them many useful dye stuffs are now prepared on the large scale. Amongst the most important of these are “neutral red,” “neutral violet,” and two other violet colouring matters, “red dyestuff,” “fuchsia,” “giroflé,” “Magdala red,” “indazine” and “Basle blue.”

Among the colouring matters before enumerated are “aniline yellow” and “Bismarck brown.” Their real nature was not understood until a few years ago; and though the use of the aniline yellow itself has been abandoned on account of its fugacity, the substance has been found a most prolific parent, which has supplied dye stuffs of the most diverse and brilliant hues. These form what chemists term the azo colours, and they have been manufactured in great variety and on a very large scale. In 1876, the class of them called chrysoidines was introduced, and again, in 1878, tropœolines. Great numbers of different azo colours have been sent into commerce under various names, such as “butter yellow,” “crocein scarlet,” “Biebrich scarlet,” “Congo red,” “Bordeaux G.,” “fast red,” &c., &c. About 140 of these azo dyes have been described, and the commercial importance of this one class of compounds alone may be inferred from the fact of no fewer than 200 patents having been taken out for processes relating to their manufacture in the eleven years from 1878 to 1888.

It would not be difficult to fill this book with instances of the way in which the resources of modern life have been increased by chemistry alone, a science almost entirely the creation of the present century. Many of the processes of manufacture in which chemistry is applied to the production of articles of every-day use have been so often described, that they may be assumed to be already so well known as to offer few elements of novelty to the general reader, whose interest would also be likely to flag if he were carried over a long range of even the brilliant discoveries that are so delightful and instructive for the special students of this science. There is no parallel to the rapidity of the progress made by the younger branch of the science which concerns itself with the chemistry of one element—namely, carbon and its various combinations, and it is from these carbon compounds that our examples have been drawn. In the explosives, we have some of these compounds supplying resistless forces for rending rocks, and furnishing in warfare the most dreadful powers of destruction. In anæsthetics, we see beneficent applications of others in alleviating suffering and annulling pain; and again we have just shown how richly another set of them can minister to our sense of beauty. The discussion of these topics has afforded an opportunity for bringing before the reader some of the laws or summarized statements of experimental facts, and also some of those symbolical conceptions of the constitution of compounds, which together furnish the clues that guide the chemist through the vast labyrinth of the endless transformations of matter. The results attained show that the notions expressed by such words as atom, molecule, compound radical, structural formula, etc., have a true representative correspondence with something in the actual constitution of bodies.

THE GREATEST DISCOVERY OF THE AGE.

The indulgent reader who may have followed the course of the foregoing pages, will perhaps peruse the title of this article with some little bewilderment. His attention has been drawn to one after another of a series of remarkable and important discoveries, and he will naturally wonder what can be the discovery which is greater than any of these. Now, a discovery is great in proportion to the extent and importance of the results that flow from it. These results may be immediate and practical, as in the case of vaccination; or they may be scientific and intellectual, as in Newton’s discovery of the identity of the force which draws a stone to the ground with that which holds the planets in their orbits. Such discoveries as most enlarge our knowledge of the world in which we live, by embracing in simple laws a vast field of phenomena, are precisely those which are most prolific in useful applications. If we admit, as we must, the truth of Bacon’s aphorism, which declares that “Man, as the minister and interpreter of nature, is limited in act and understanding by his observation of the order of nature; neither his understanding nor his power extends farther,”[19] then it would be easy to show that the discovery of which we have to treat, more than any other, must be of immense practical service to mankind in every one of the ways in which a knowledge of the order of nature can be of use, viz.:—“First, In showing in how to avoid attempting impossibilities. Second, In securing us from important mistakes in attempting what is, in itself, possible, by means either inadequate or actually opposed to the end in view. Third, In enabling us to accomplish our ends in the easiest, shortest, most economical, and most effectual manner. Fourth, In inducing us to attempt, and enabling us to accomplish, objects which, but for such knowledge, we should never have thought of undertaking.”[20]

Footnote 19:

“Homo naturæ minister et interpres, tantum facit et intelligit quantum de naturæ ordine re vel mente observaverit: nec amplius scit aut potest.”—Novum Organum, Aphor. I.

Footnote 20:

Sir J. Herschel.

A great principle, like that which we are about to explain to the reader, is too vast in its bearings for its discovery and elaboration to have been the work of an individual. This truth, and indeed the whole of our knowledge, is but the result of the development and growth of pre-existing knowledge. In fact, every discovery, however brilliant—every invention, however ingenious, is but the expansion or improvement of an antecedent discovery or invention. In strictness, therefore, it is impossible to say where the first germ of even our newest notions may be found. Our latest philosophy can be shown to be the result of progressive modifications of ideas of remote ages. Hence every great truth, every grand invention, has in reality been the offspring of many minds; but we record as the discoverers and inventors those men who have made the longest strides in the path of progress, and whose genius and labours have overcome obstacles defying ordinary efforts.

The extent of the field which is covered by the principle we have in view is so vast—embracing, as it does, the whole phenomena of the universe—that it will not be possible to do more within our limits than give the reader a general notion of the principle itself. It may be useful to instance a truth which has a similar generality and significance, and which has also acquired the force of an axiom, because it is verified every hour. It is that greatest generalization of chemistry, affirming that in all its transformations matter is indestructible, and can no more be destroyed than it can be called into being at will. This truth is so well established, that some philosophers have asserted that an opposite state of things is inconceivable. But it was not always known; and there are at the present day untutored minds which not only believe that a substance destroyed by fire is utterly annihilated, but what they find inconceivable is the continued existence of the substance in an invisible form. The candle burns away, its matter vanishes from our view; but if we collect the invisible products of the combustion, we find in them the whole substance of the candle in union with the atmospheric oxygen. We may, in imagination, follow the indestructible atoms of carbon in their migrations, from the atmosphere to the plant, which is eaten by the animal and goes to form its fat, and from the tallow, by combustion, back into the atmosphere again. The notion of the real identity of matter under changing forms has been expressed by our great dramatist in a well-known passage, which is remarkable for its philosophic insight, when we consider the age in which it was written:

HAMLET. To what base uses we may return, Horatio! Why may not imagination trace the noble dust of Alexander, till he find it stopping a bung-hole?

HORATIO. ‘Twere to consider too curiously to consider so.

HAMLET. No, faith, not a jot; but to follow him thither with modesty enough, and likelihood to lead it. As thus: Alexander died, Alexander was buried, Alexander returneth to dust; the dust is earth; of earth we make loam; and why of that loam, whereto he was converted, might they not stop a beer-barrel?

Imperial Cæsar, dead, and turned to clay, Might stop a hole to keep the wind away; O, that the earth, which kept the world in awe, Should patch a wall to expel the winter’s flaw!

Now the greatest discovery of our age is that force, like matter, is indestructible, and that it can no more be created than can matter. The reader may perhaps think the statement that we cannot create force is in contradiction to experience. He will be disposed to ask, What is the steam engine for but to create force? Do we not gain force by the pulley, the lever, the hydraulic press? And are not tremendous forces produced when we explode gunpowder or nitro-glycerine? When the principle with which we are here concerned has been developed and stated in accurate terms, it is hoped the reader will see the real nature of these contrivances. We are, however, aware that it is quite impossible within the limits of a short article to do much more than indicate a region of discovery abounding with results which may be yet unfamiliar to some. Into this, if so minded, they should seek for further guidance, which they will pleasantly find in the pages of Dr. Tyndall’s “Heat considered as a Mode of Motion,” and in a little work by Professor Balfour Stewart, entitled “The Conservation of Energy,” and quite fascinating from the clearness and simplicity of its style. We may continue our humble task of merely illustrating the general nature of this, in reality the most important, subject which we have had occasion to bring under the reader’s notice.

Perhaps the first step should be to point out the fact of the various forces of nature—mechanical action, heat, light, electricity, magnetism, chemical action—being so related that any one can be made to produce all the rest directly or indirectly. Some examples of the conversion of one form of force into another occur in the foregoing pages. Thus, on page 485 an experiment is described in which electricity produces a mechanical action; electricity is also shown, on page 496, to produce heat; on page 491 chemical action; on page 501 magnetism. Then, as instances of the inverse actions, there is on page 488, in the first paragraph on “Electric Induction,” an account of the mode in which mechanical movements may give rise to electricity; and in the experiments in pages 508, 509, and particularly in the account of the Gramme machine, page 511, it is shown how mechanical movements can, through magnetism, produce electricity. The voltaic element, page 491, and the galvanic batteries, are instances of chemical action supplying electricity. On page 518 a striking instance is mentioned of changes in the forms of force. Every lighted candle is a case of chemical action giving rise to light; and interesting examples of the inverse relation are referred to on page 608. On page 168 is represented the conversion of arrested motion into heat and light. We have, indeed, sufficient examples to arrange a series of these conversions of forces in a circle. Thus, chemical action (oxidation in the animal system) supplies muscular power, this sets in motion a Gramme machine, the motion is converted into electricity, the electricity produces the electric light, and light causes chemical action, and with this the cycle is complete. In the steam engine heat is converted into mechanical force, and many cases will present themselves to the reader’s mind in which mechanical actions give rise to heat. The doctrine of a mutual dependence and convertibility among all the forms of force was first definitively taught in England by Mr. (now Justice) Grove, in 1842; and almost simultaneously Dr. Meyer promulgated similar views in Germany. Mr. Grove subsequently embodied his doctrine in a treatise, called “The Correlation of the Physical Forces,” which has seen several editions.

But this teaching included much more than a mere connection between the various forces, for it extended to quantitative relations. It declared that a given amount of one force always produced a definite amount of another, that a certain quantity of heat, for example, would give rise to a certain amount of mechanical action, and that this amount of mechanical action was the equivalent of the heat which produced it, and would in its turn reproduce all that heat. These last doctrines, however, rested on a speculative basis, until Mr. James Prescott Joule, of Manchester, carried out a most patient, laborious, and elaborate experimental investigation of the subject. His labours placed the truth of the numerical equivalence of forces on a foundation which cannot be shaken; and he accomplished for the principle of the indestructibility of force what Lavoisier did for that of the indestructibility of matter—he established it on the incontrovertible basis of accurate and conclusive experiment. His determination of the value of the mechanical equivalent of heat especially is a model of experimental research; and subsequent investigators have, by diversified methods, confirmed the accuracy of his results. A great part of his work consisted in finding what quantity of heat would be produced by a given quantity of work.

Before we proceed to give an indication of one of Dr. Joule’s methods of making this determination, we may point out that if a weight be raised a certain height, the work which is done in raising it will be given out by the weight in its descent. If you carry a 1 lb. weight to the top of the London Monument, which is 200 ft. high, you perform 200 units of work. When the weight is at the top, the work is not lost; for let the weight be attached to a cord passing over a pulley, and it will, as it descends, draw up to the top another 1 lb. weight.[21] If you drop the weight so that it falls freely, it descends with a continually increasing velocity, strikes the pavement, and comes to rest. Still your work is not lost. The collision of the weight and the pavement develops heat, just as in the case of the experiment depicted on page 168, but to a less degree—the increase of temperature might not be sensible to the touch, but could be recognized by delicate instruments. Your work, then, has now changed into the form of heat—the weight and the pavement are hotter than before. This heat is carried off by contiguous substances. But still your work is not lost, for it has made the earth warmer. The heat, however, soon flows away by radiation from the earth, and is diffused into space. The final result of your work is, then, that a certain measurable quantity of heat has been sent off into space. Is your work now finally lost? Not so: in reality, it is only diffused throughout the universe in the form of radiant heat of low intensity. Yet it is lost for ever for useful purposes; for from this final form of diffused heat there is no known or conceivable process by which heat can be gathered up again.

Footnote 21:

See Note B, at the end.

Dr. Joule arranged paddles of brass or iron, so that they could turn freely in a circular box containing water or quicksilver. From the sides of the box partitions projected inwards, which contained openings that permitted the divided arms of the paddle to pass, and preventing the liquid from moving en masse, thus caused a churning action when the paddle was turned. Now, every one who has worked a rotatory churn knows that a considerable resistance is offered to this action; but every one does not know that under these circumstances the liquid becomes warmer. It was Dr. Joule’s object to discover how much the temperature of his liquid was raised by a measured quantity of work. He used very delicate thermometers, and had to take a number of precautions which need not here be described; and he obtained the definite quantity of work by the descent of a known weight through a known distance, a cord attached to the weight being wound on a drum, which communicated motion to the paddle. The experiments were conducted with varying circumstances, to avoid chances of error, and were repeated very many times until uniform and consistent indications were always obtained. The result of the experiments showed that 772 units of work (foot-pounds) furnished heat which would raise the temperature of 1 lb. of water from 32° to 33° F., which is the unit of heat. This number, 772, is a constant of the greatest importance in scientific and practical calculations, and is called “the mechanical equivalent of heat.” The amount of work it represents is sometimes called a “Joule,” and is always represented in algebraical formulæ by “J.” Mr. Joule’s first paper appeared in 1843, and soon afterwards various branches of the subject of “The Equivalence and Persistence of Forces” were taken up by a number of able men, who have advanced its principles along various lines of inquiry. Among the most noted contributors to this question we find the names of Sir William Thomson, Helmholtz, James Thomson, Rankin, Clausius, Tait, Andrews, and Maxwell.

In the steam engine the case is the inverse of that presented by the above-named experiment of Dr. Joule’s. Here we have heat producing work. Now, the quantity of steam which enters the cylinder of a steam engine may be found, and the temperature of the steam can be determined, and from these the amount of heat which passes into the cylinder per minute, say, can be calculated. A large portion of this heat is, in an ordinary engine, yielded up to the condensing water, and another part is lost by conduction and radiation from the cylinder, condenser, pipes, &c. But both these quantities can be estimated. When the amount is compared with that entering the cylinder in the steam, a difference is always found, which leaves a quantity of heat unaccounted for. When this quantity is compared with the work done by the engine in the same interval (which work can be measured as described on page 10), it is always found that for every 772 units of work a unit of heat has disappeared from the cylinder. The numerical relation between work and heat which is established in these two cases has been tested in many quite different ways; and, within the limits of experimental errors, always with the same numerical result. But equally definite quantitative relations are known to exist among all the other forms of force; and the manner in which these are convertible into each other has already been indicated, although want of space prevents full illustration of this part of the subject. It may, however, be seen that each form of force can be mediately or immediately converted into mechanical effect, hence each is expressible in terms of work. That is to say, we can assign to a unit of electricity, for example, a number expressing the work which it would do if entirely converted into work; and the same number also expresses the work which would be required to produce the unit of electricity. An ounce of hydrogen in combining with 8 oz. of oxygen produces a certain measurable quantity of heat. If that heat, say = H, were all converted into work, we now know that the work would = HJ. Hence we can express a definite chemical action in terms of work. The same is generally true of all physical forces, though in some cases, such as light, vital action, &c., the quantitative relations have not yet been definitely determined.

Since, then, all the forces with which we are acquainted are expressible (though the exact relations of some have yet to be discovered) in terms of work, it is found of great advantage to consider the power of doing work as the common measure of doing all these. Thus, if we define energy as that which does, or that which is capable of doing, work, we have a term extremely convenient in the description of some aspects of our subject. Thus we can now speak of the energies of nature, instead of the forces. And all forces, active or passive, may be summed up in one word—energy. And, further, the great discovery of the conservation of forces under definite equivalents, may be summed up very briefly in this statement—THE AMOUNT OF ENERGY IN THE UNIVERSE IS CONSTANT. To make this statement clear requires that a distinction between two forms of every kind of energy be pointed out. To recur to the example before imagined: if you carry the 1 lb. weight up the Monument, and deposit it on the ledge at the top, it might lie there for a thousand years before it was made to give back the work you had performed upon it. That work has been, in a manner, stored up by the position you have given to your weight. Now, in taking up the weight, you expended energy—you really performed work: that is an instance of energy in operation, and may be termed “actual energy.” In what form does the energy exist during the thousand years we may suppose your weight to lie at the top of the Monument? It is ready to yield up your work again at any moment it is permitted to descend, and it possesses therefore during the whole period a potential energy equal in amount to the actual energy you bestowed upon it. A similar distinction between actual and potential energy exists with regard to every form of force. If by any means you separate an atom of carbon from an atom of oxygen, you exert actual energy. The process is analogous to carrying up the weight. The atoms when separated possess potential energy,—they can rush together again, like the weight to the earth, and in doing so will give out the work which was expended on their separation. A parallel illustration might be drawn from electrical force.

A typical example of the storing up of energy is furnished by a crossbow. The moment a man begins to bend the bow he is doing work, because he pulls the string in opposition to the bow’s resistance to a change in its form; and it is plain that the amount of energy thus expended is measurable. Suppose, now, the bow has been bent and the string caught in the notch, from which it is released by drawing the trigger when the discharge of the bow is desired. The bow may be retained for an indefinite period in the bent condition, and in this state it possesses, in the form of potential energy, all the work which has been expended in bending it, and which it will, in fact, give out, in some way or other, whenever the trigger is drawn. To fix our ideas, let us suppose that to draw the string over the notch required a pull of 50 lbs. over a space of 6 in.; that is equivalent to 50 × ½ = 25 units of work. Now let the bow be used to shoot an arrow weighing ¼ lb. vertically upwards. The height in feet to which the arrow will rise multiplied into its weight in pounds will be the work done upon it by the bow. Now, we say that experiment proves that in the case supposed the arrow would rise just 100 ft., so that the work done by the bow (¼ × 100 = 25) would be precisely that done upon it. For the sake of simplicity, we keep this illustration free from the mention of interfering causes, which have to be considered and allowed for when the matter is put to the real test of quantitative experiment. The instance of the crossbow brings into notice a highly instructive circumstance, which is this: the bow, which it may have taken the strength of a Hercules to bend, will shoot its bolt by the mere touch of a child on the trigger. In the same way, when a man fires a gun, he merely permits the potential energy contained in the charge to convert itself into actual, or kinetic, energy. The real source of the energy, in the case of the child discharging the crossbow, is the muscular power of the man who drew it; the real source of the energy in exploding gunpowder is the separation of carbon atoms from oxygen atoms, and that has been done by the sun’s rays, as truly as the string was pulled away from the bow by muscular power. If we turn our attention to nitro-glycerine or to nitro-cellulose, we can, by following the chemical actions giving rise to these substances, in like manner trace their energies to our great luminary. The unstable union by which oxygen and nitrogen atoms are locked up in the solid and liquid forms of nitro-cellulose and nitro-glycerine is also the work of the sun; for nitrogen acids, or rather nitrates, are produced naturally under certain electrical and other conditions of the atmosphere, which are due, directly or indirectly, to the sun’s action; and they cannot be formed artificially, except by imitating the natural conditions, as by passing electric sparks through air, &c.

It will now be understood, as regards the wonderful relations between animal and vegetable life, which have already been alluded to more than once, how the sun, by expending actual energy, separates atoms of carbon from atoms of oxygen in the leaves of plants, and confers upon these a position of advantage, i.e., potential energy; and how animals, absorbing the separated carbon in the form of food, and inhaling the separated oxygen in the air they breathe, cause the conversion of the potential into actual energy, which appears in the heat, movements, and vital functions of the animal body. In coal we have the energy which plants absorbed from the sun ages ago, stored up in a potential form. The carbon atoms are ready to rush into union with oxygen atoms, and convert their energy of position into the energies developed by chemical action, viz., heat, light, &c. Energy is thus constantly shifting its form from actual to potential, and vice versâ, and exhibiting itself under the various transformations of force, as when sun-force changes to chemical action, chemical action to heat, heat to electricity, &c. Energy is, indeed, the real modern PROTEUS—constantly assuming different shapes, difficult to grasp if not held in fetters; now taking on the form of a lion, now of a flame of fire, a whirlwind, a rushing stream. As sober, literal matter of fact we catch glimpses of energy under these very forms.

The greatest discovery of the age has, as already indicated, immediate and important practical bearings. The amount of thought which, even in the present day, is devoted by unscientific mechanics to the old problem of perpetual motion is far greater than is generally supposed. The principle of the conservation of energy shows that this is an impossibility; that the inventor who seeks to create force might just as well try to create matter; that the production of a perpetually moving self-sustaining machine is as far removed from human power as the bringing into existence of a new planet. In force, as in matter, the law is inexorable—ex nihilo nihil fit. Again, knowing the definite amount of energy obtainable from the combustion of a pound of coal, we can compare the amount we actually procure from it in our steam engines with this theoretical quantity as the limit towards which our improvements should bring us continually nearer, but which we can never exceed, or, indeed, even reach. The schemers of perpetual motion are not the only class of speculators who pursue objects which are incompatible with our principle. There are many who seek to accomplish desirable ends by inadequate means: who, for example, are aiming perhaps to accomplish the reduction of ores by a quantity of fuel less than that mechanically equivalent to the work, or who conceive that by adding to coal some substance which itself is unchanged, an indefinitely greater amount of heat may be liberated by the combustion.

Enough has been said to show that the energies of animal life can be traced to the sun as their source. The sun builds up the plant, separating oxygen from carbon. The animal—directly or mediately by devouring other animals—takes the carbonaceous matter of the plant, and reunites it with oxygen. In the plant the sun winds up the spring which gives life to the animal mechanism; for the winding-up of a spring and the separation of the atoms having chemical affinities are alike instances of supplying potential energy. In the animal there is a running-down of the potential into actual energy. It is plain also that of the total energy radiated from the sun in every direction, the earth receives but a very small part (1/2300000000). By far the larger part is diffused into space, where, for all such purposes as those with which we are concerned, it is lost. The heat which the sun sends out in a year is calculated to be equal to that which would be produced by the combustion of a layer of coal 17 miles thick over the whole surface of the luminary. Is the sun, then, a flaming fire? By no means. Combustion is not possible at its temperature; and as we know the substances which enter into its composition are the same as those we find in the earth, we know that the chemical energies of such substances could not supply the sun’s expenditure. Passing over as unsatisfactory an explanation which might occur to some minds—namely, that the sun was created hot at the beginning, and has so continued—there are two theories which attempt to account for the sun’s heat. One is that of Meyer, who supposes the heat is due to the continual impact of meteorites drawn to the sun by its gravity; and the other is that of Helmholtz, who attributes the heat to the continual condensation of the substance of the sun. Helmholtz calculates that a shrinking of the sun’s diameter by only 1/10000th of its present amount, would supply heat to last for two thousand years; while the condensation of the substance of the sun to the density of the earth would cover the sun’s expenditure for 17,000,000 of years. There is great probability that both theories may be correct, and that the cause of the sun’s heat may be considered as due in general terms to aggregation of matter, by which the original potential energy of position is converted into the actual energy of heat and light. Now, however immense may be our planetary system, the sun being continually throwing off this energy into space, there must come a time when the supplies of meteorites will fail, and when the great globe of the sun will have shrunk to its smallest dimensions. We see, then, that heat and light are produced by the aggregation of matter; the heat and light are radiated into space; the small fraction intercepted by our globe is the source of almost every movement—the original stuff, so to speak, out of which all terrestrial forces are made. The sun produces the winds, the thunderstorms, the electric currents of the Aurora, the phenomena of terrestrial magnetism, and is the source of vegetable and animal life. The waves, the rains, the mountain torrents, the flowing rivers, are the work of the sun’s emanations.

In the illustration of the energy expended on raising a weight afterwards dropped, we traced that energy into the final form of heat of a low temperature radiated into space. It would be easy to show that all energy ultimately takes the same form. Now, although it is easy to convert work into heat, there is no conceivable process by which uniformly-diffused heat can again be made to do any kind of work. The case may be compared to water, which in moving down from a higher to a lower level may be made to perform any variety of work. But when all the water has passed down from the higher level to the lower, it can no longer do any work. Whenever work is done by the agency of heat, there is always a passing from a higher temperature to a lower—a transference of heat from a hotter body to a colder. If the condenser of the steam engine had the same temperature as the steam, the machine would not work. Not only do all the energies in operation on the face of the earth continually run down into the form of radiant heat sent off by the earth into space; but our sun’s energy, and that of the suns of other systems, are also continually passing off into space; and the final effect must be a uniform diffusion of heat in a universe in which none of the varied forms of energy we now behold in operation will be possible, because all will have run down to the same dead level of uniformly-diffused heat. This startling corollary from the principle of the conservation of energy has been worked out by Sir W. Thomson under the title of “The Dissipation of Energy.” It leads us to contemplate a state of things in which all light and life will have passed away from the universe—a condition which the poet’s terrible dream of darkness, “which was not all a dream,” seems to shadow forth—

“The bright sun was extinguished, and the stars Did wander darkling in the eternal space, Rayless and pathless; and the icy earth Swung blind and blackening in the moonless air.

* * * * *

The world was void, The populous and the powerful was a lump, Seasonless, herbless, treeless, manless, lifeless— A lump of death—a chaos of hard clay. The rivers, lakes, and ocean all stood still, And nothing stirred within their silent depths.

* * * * *

The waves were dead; the tides were in their grave, The Moon, their mistress, had expired before; The winds were withered in the stagnant air, And the clouds perished; Darkness had no need Of aid from them—She was the Universe.”

The doctrine of this persistence and dissipation of energy completely harmonizes with the grand speculation termed the “nebular hypothesis,” which regards the universe as having originally consisted of uniformly diffused matter, which, being endowed with the power of gravitation, aggregated round certain centres. This process is still going on; and, according to modern speculations, light and life and motion are but manifestations of this primæval potential energy being converted into actual energy, and degrading ultimately into the form of universally-diffused heat. To quote the closing sentences of the eloquent passage in which Professor Tyndall concludes the work mentioned above, “To nature nothing can be added, from nature nothing can be taken away; the sum of her energies is constant, and the utmost man can do in the pursuit of physical truth, or in the applications of physical knowledge, is to shift the constituents of the never-varying total. The law of conservation rigidly excludes both creation and annihilation. Waves may change to ripples, and ripples to waves; magnitude may be substituted for number, and number for magnitude; asteroids may aggregate to suns, suns may resolve themselves into floræ and faunæ, and floræ and faunæ melt in air: the flux of power is eternally the same. It rolls in music through the ages, and all terrestrial energy—the manifestations of life as well as the display of phenomena—are but the modulations of its rhythm.”

The discoveries to which we have here endeavoured to attract the reader’s attention thus give rise to conceptions of the utmost grandeur and interest. We see that the sum of Nature’s energies is constant; that all the manifestations of force are but the transference of power from one position to another. And we have recognized the material source of all our terrestrial energies in the sun. Two theories have already been mentioned by which it is sought to account for the sun’s heat—the meteoric theory of Meyer and Thomson, and the shrinkage theory of Helmholtz. These both assume gravitation as the primal force from which the supply of heat and other energies must be drawn, and they assume also that the laws of radiation and of the degradation of temperature in the transformation of heat into other forces, as we find them operating at the earth’s surface, are equally in action in every region of space. Hence is deduced that conception of the final state of the universe as one of merely equally diffused temperature admitting of no further transformation. This speculation presents the universe in the aspect of a clock, now indeed going, but when once run down, incapable of ever being again wound up. There seems in this view a want of symmetry, so to speak; we miss the feeling of harmonious rhythm to which Tyndall refers. There is, however, another cosmic theory, well supported by accumulating facts, which assigns to gravitation a less important part in the production of solar heat and in the evolution of worlds, and it is one which supplies also a basis for the explanation of such phenomena as aerolites, comets, variable stars, the inclination of planets’ axes to their orbits, the proper motion of our sun, and that of the so called fixed stars, of all of which the nebular hypothesis fails to give any account; while, on the other hand, the impact theory, as it has been named, includes the other, and goes beyond it. The reader who desires to pursue this subject may be referred to Croll’s book on Stellar Evolution.

In the last few paragraphs we have been dealing with speculations as much as with discoveries. But indeed the former are the offspring of the latter, as certainly as one invention becomes the parent of others. The human mind never rests contented with the knowledge and mastery of nature actually gained, but ever seeks to pass beyond and attain still greater power. The volume we are now bringing to a close has given but brief and imperfect indications of specimens, taken here and there, of what has been done during the short period of one century. We may draw an augury for the future of man’s dominion from the powers his Promethean spirit has already grasped:

“The lightning is his slave; heaven’s utmost deep Gives up her stars, and like a flock of sheep They pass before his eye, are numbered, and roll on! The tempest is his steed, he strides the air; And the abyss shouts from her depth laid bare, “Heaven, hast thou secrets? Man unveils me; I have none.”

NOTES A AND B.

Note A—Continuation of Table on page 755, showing the quantity of Coals raised annually in Great Britain.

Year. Coal raised in Tons. 1874 126,590,108 1875 133,306,458 1876 134,125,166 1877 134,179,968 1878 132,612,063 1879 133,720,393 1880 146,969,409 1881 154,184,300 1882 156,499,977 1883 163,737,327 1884 160,757,779 1885 159,351,418 1886 157,518,482 1887 162,121,576 1888 169,935,219 1889 176,916,724 1890 181,614,280 1891 185,479,126 1892 181,786,871 1893 164,325,795 1894 188,277,525

Note B—CONSERVATION OF ENERGY.—Page 804.

The statement here should have been more explicit, as it has reference to a state of things not to be realised in practice. Like the well-known “first law of motion,” it can neither be demonstrated à priori, nor proved by any direct and simple experiment. The first law of motion asserts that a body in motion, not acted on by any external force, will continue to move in a straight line, and with a uniform velocity. Now we cannot place a body in such a position that it will not be acted upon by some external forces; but the more we lessen the effect of external forces, the more nearly is the motion straight and uniform. Similarly in the case supposed, the intention is to show that the weight carried up is in a position to do just as much work as was done upon it. We must suppose several impracticable but conceivable conditions in order to eliminate considerations which do not concern the theoretical question; we must suppose the cord to be weightless and absolutely devoid of rigidity; the pulley to have no mass or inertia, that is to require no force to set it in motion, and to move without any friction; the air to offer no resistance; and the force of gravity to be uniform throughout the space. Some approximation to these conditions is practicable, as, for example, the pulley might be the lightest possible, and turn on friction wheels, the cord might be the finest silk thread, and so on. But it is not the influence of these external forces we are considering, but only the energy due to the position of the raised weight. Assuming, therefore, the disturbing conditions absolutely eliminated, it is not difficult to see that no downward force or pressure, however small, could be applied for ever so short a time, to the upper weight without setting the system in motion. The motion would be an accelerated one so long as the force was applied, it would become uniform when the force ceased to act; it would have a velocity proportionate to the force. In any case, after a time the descending weight would reach the ground, and for our point of view it is quite immaterial whether the time occupied by the movement were 5 minutes or 5,000 years, for be it observed, time does not enter into the definition of work as it does into that of “horse-power.” Then by pushing the conceived conditions to their limits, we may see that without considering any question of conversion of motion into heat, the raised weight can, in theory at least, give back again the energy spent upon it.

INDEX

A.

Abel, Professor, 746.

Accumulators, 530.

Adhesion of locomotive, 21.

Advantages of present age, 2.

Aerolites, 30.

Air, 734.

Albert Bridge, Saltash, 283.

Alizarine, 797.

“Alliance” magneto-electric machine, 520.

Aluminium, 717. bronze, 719. reduced cost of, 723.

American Tract Society building, 78.

Ampère’s hypothesis, 750. rule, 492, 549.

Amphioxus, 679.

ANÆSTHETICS, 731.

Anemometer, 656.

Angle, limiting, or critical, 399.

Aniline, 787. black, 793. blue, 790. green, 791. purple, 788.

Anomalous magnetisation, 538.

Anthea Cereus, 678.

Anthracene, 796, 797.

Applegath printing machine, 312. and Cowper, ditto, 308.

Apps’s anemometer, 656. induction coil, 506.

AQUARIA, 675.

Arago, 599.

Architecture, use of iron in, 72.

Argand gas burners, 773. lamps, 595.

Armours, ships’, strengths of, 166.

Armstrong 110–ton gun, 202.

Armstrong’s guns, 192. hydraulic crane, 333.

Atoms, 733, 743.

Aurora, 504.

Australian gold, 688.

Austrian torpedoes, 229.

“Automobile” competition, 23.

Axolotl, 686.

B.

Bacon, Francis, 799.

Balloon, photographic, 628.

Basic process (Gilchrist’s), 66.

Battery, galvanic, 493.

Battery, secondary, 530.

Baxter House experiments, 58.

Bell Rock Lighthouse, 593.

Bells, electro-magnetic, 554.

Benzol, 783.

Bessemer, 26, 56. Channel steamer, 142. converter, 63. iron, 62. process, 64. steel, 56–67.

BIG WHEELS, 81.

Bitter Lakes, 261.

BLANCHARD LATHE, THE, 86.

Blast furnace, 40.

Blind spot in eye, 460.

Blister steel, 54.

Blood spectra, 431.

Boilers of steam engines, 13.

Boring for coals, 361.

Bourdon’s pressure gauge, 12.

Bourseul, M., 582.

Box girders, 280.

Breakwater, 258.

Breakwaters for Suez Canal, 258.

BREECH-LOADING RIFLES, 182.

Brewster, Sir D., 405, 420, 470, 474.

Bridge, projected Channel, 296.

Bridgewater Canal, 250, 266.

BRIGHTON AQUARIUM, 682.

Britannia Bridge, 280. raising tubes, 336.

British Aluminium Co., 723.

British navy in 1894, 167.

BROOKLYN BRIDGE, 303.

“Brown Bess,” 178.

Browning’s micro-spectroscopes, 434. spectroscope, 422, 432.

Brunel, 283.

Brunswick rifle, 180.

Brush dynamo, 522.

Bullet, Greener’s expanding, 182. Lebel, 188. Minié, 180.

Bullets, machinery for making, 330.

Bunsen and Kirchhoff, 422. and Roscoe, 720.

Bunsen’s battery, 496. burner, 421.

Bye-products in gas making, 772.

C.

Cable railways, 126.

Cæsium, 426.

Caissons for Forth Bridge, 293.

Calais-Douvres, steamship, 141.

Caledonian Canal, 250.

Calico printing machines, 321.

California, discovery of gold in, 688.

Camera, 613.

Canal, Caledonian, 250. Manchester Ship, 262. Nicaragua, 274. North Sea, 271. Panama, 272. Suez, 251.

CANTILEVER BRIDGES, 291.

“Cape Horn,” 120.

Captain, H.M.S., 141.

Carbon transmitter, 590.

Carbonic oxide, 44, 48.

Carbons for arc lights, 527.

Carcel lamp, 597.

Carpenter, Dr. W. B., 462.

Carriages, railway, 111. for rock drills, 358.

Carriers in pneumatic tubes, 344.

Cars, Pullman, 112.

Castalia, steamship, 139.

Cast-iron, composition of, 43.

Cast steel, 54.

Catoptric lighthouse apparatus, 599.

CAUSE OF LIGHT AND COLOUR, 408.

CELESTIAL CHEMISTRY AND PHYSICS, 436.

Celluloid, 622.

Central Telegraph Office, London, 574.

Centres of gravity and buoyancy, 149.

Centrifugal force, 107.

Chains, 330.

Chain-testing machine, 329.

Channel Bridge (projected), 296. steamers, 142.

CHANNEL TUNNEL, 364.

Chassepot rifle, 182.

Chemical action of light, 608. equations, 734. nomenclature, 782. symbols, 733. work of electricity, 497.

Chloroform, 608.

Chromatic aberration of eye, 462.

Chromo-lithography, 638.

Chronograph}

Chronoscope} electric, 656.

Cincinnati Suspension Bridge, 287.

City of Rome, steamship, 139.

Clark’s hydraulic lift graving dock, 331.

Clarke’s magneto-electric machine, 509.

Clay process, stereotyping by, 633.

Clerk Maxwell’s theory of light, 541.

“Clermont,” the, 147.

Clifton Suspension Bridge, near Bristol, 285. Niagara, 287.

COAL, 751.

Coal in Kent, 371.

COAL-GAS, 764.

COAL-TAR COLOURS, 781.

Code, telegraphic, of American War Department, 528. Morse’s, 560. Wheatstone’s dot, 565.

Cold-short iron, 62.

Colesberg, 703.

Collodion process, 618.

Colour printing, 639.

Colours not in the objects, 413. photography of, 628.

Comets, spectra of, 444.

Composition rollers, 406.

Condie’s steam-hammer, 28.

Copying principle, 86.

Cordite, 748.

Corona, 438.

Cort’s puddling furnace, 45.

Couple, mechanical, 149.

Cramp gauge, 129.

Croll, on Stellar Evolution, 810.

Crookes, 507.

Crystal Palace, an example of use of iron in architecture, 72.

Crystal Palace, 72. AQUARIUM, 677.

Crystalline lens, 455.

Cup and cone, 49.

Current, electric, 492. induced, 502. measurement of, 536.

Currents in submarine cables, 579.

D.

Daguerre, 609.

Daguerreotype, 610.

Daimler motor, 24.

Dallmeyer, 617.

Daniell’s battery, 495.

De Beers Mines, 707.

Delphi, oracle at, 739.

Dial telegraphs, 566.

Diamond cutting, 700.

Diamond, qualities of the, 698. rock drill, 359.

Diamondiferous, 703.

Diamonds, 696. use of, 701.

Dioptric lighthouse apparatus, 600.

Discoveries, progressive, 802.

Dissipation of energy, 808.

Distinct vision, 458.

D lines of sodium spectrum, 425, 441.

Dolls, talking, 674.

Domestic consumption of coal, 755, 756.

DOUBLE REFRACTION AND POLARIZATION, 399.

Dredges, Suez Canal, 255.

Drilling machine, 90.

Dry digging, 702.

Duboscq’s electric lamp, 497.

Du Moncel, 590.

DYNAMICAL ELECTRICITY, 490.

Dynamo, Siemens’, 522. Brush, 522.

E.

Earl’s Court, Great Wheel at, 83.

Earth’s circuit, 574.

Ebonite, 728.

Eccentric, 9.

Eclipse of sun, 438.

Eddystone lighthouse, 594.

Edison, 669, 670, 674.

Edison’s kinetoscope, 478.

Eiffel Tower, the, 72.

ELECTRIC LIGHTING AND ELECTRIC POWER, 519.

Electric current, 790. furnace, 722.

ELECTRIC INDUCTION, 488. launch, 534. light, cost of, 43.

Electric light in lighthouses, 515. telegraph, 598. torpedo, 547. tramway, 532. welding, 537.

ELECTRICITY, 481.

ELECTRICITY, THE NEW, 538.

Electrode, 497.

Electro-magnet, 500.

Electromotive force, 494.

Electro-plating, 499, 518.

Electrotyping, 634.

Elementary bodies, 716.

ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY, 483.

Elswick 4·7–in. gun, 206. guns, 194.

Energy, 806.

Ether, 735. the luminiferous, 408.

Exhaustion of coal, 755, 756, 757.

Expansive working of steam, 8, 17.

Explosion by concussion, 745. of locomotive, 21. of torpedoes, 229.

EXPLOSIVES, 225, 740. different effects of, 748. names and classes of, 750.

EYE, THE, 451. dimensions of some parts of, 462.

Eye not optically perfect, 462.

Eyeballs, muscles of, 461.

F.

Fairbairn, Sir W., 280.

Faraday, 506, 508, 735. ventilating gas-burner, 773.

Faure’s accumulator, 530.

Fellahs, 255.

Ferris wheel, Chicago, 81.

Field telegraphs, 555.

FIRE-ARMS, 169.

Fish-plates, 105.

Fizeau, 386.

Floating matter in air, 383.

Fluids, electric, 487.

Fly-wheels, 7.

Force, conservation of, 804. electromotive, 494.

FORTH BRIDGE, THE, 311.

Foucault, 387.

Fovea centralis, 456, 457.

Fraser-Woolwich guns, 195.

Fraunhofer’s lines, 420, 436.

Fresnel’s mirrors, 409. measurement of velocity of light, 600.

Fribourg Suspension Bridge, 286.

Froment’s dial telegraph, 567.

Furnace, electric, 322.

G.

Galvanic batteries, 493, 494.

Galvanometer, 493. mirror, 570.

Gas engine, 25. governor, 769. holder, 766. making apparatus, 765. meters, 775. pressure, 769. retorts, 766.

Gases of blast furnace, 49.

Gatling battery gun, or mitrailleur, 219.

Gauge, broad and narrow, 106. Bourdon’s pressure, 12.

Geissler’s tubes, 505.

Ghost, Pepper’s, 392.

Giffard’s injector, 11.

GIRDER BRIDGES, 280.

Glass, strains in, 407.

Glatton, H. M. S., 161.

Glynde, electric railway, 534.

GOLD, 686.

GOLD AND DIAMONDS, 687.

Gold-mining operations, 690.

Goodyear, Mr., 727.

Governor of steam engines, 6.

Gower Street Station, 114.

Gramme magneto-electric machine, the, 511.

Graphophone, 672.

Graphotype, 644.

Gray, 590.

GREAT BROOKLYN BRIDGE, 303.

Great Eastern, 133, 152, 330, 465, 578.

GREATEST DISCOVERY OF THE AGE, 801.

Greener’s expanding bullet, 182.

Grove, Sir W. R., 804.

Grove’s battery, 495.

Gun, 32–pounder, 191. 68–pounder, 192. 35–ton, 201. 81–ton, 201. 100–ton, 201. 110–ton, 202. Elswick 4·7–in., 206. Maxim, 225. Moncrieff, 208. Nordenfelt, 223.

Gun-cotton, 747. torpedoes, 233.

Gunpowder, 734.

Guns, Armstrong’s, 192. Elswick, 194. Fraser-Woolwich, 195. Krupp’s, 214. quick-firing, 206. submarine, 240.

GUTTA-PERCHA, 728.

H.

Half-tone process, 629.

Hancock, Mr. Charles, 729. Mr. Thomas, 725.

Harvey’s torpedoes, 234.

Heat produced by electric current, 502.

Heat spectrum, 613.

Heating by gas, 776.

Helmholtz, 462, 464, 472, 474.

Henry, on Leyden jar discharge, 538.

Hercules, H.M.S., 150.

Hertz, Professor, 541.

Hippocampus, 664.

Hoe’s printing machines, 316, 318.

Holmes’ magneto-electric machine, 520.

Holophotal light, 604.

Holyhead and Kingston steamers, 136.

Horseless carriages (Automobiles), 23.

Horse-power, 10.

Hot-blast, 48.

Hotchkiss quick-firing guns, 208.

Hough’s metereograph, 654.

Howitzers, 213.

Hudson River steam navigation, 147.

Hughes’ printing telegraph, 560. microphone, 590.

HYDRAULIC POWER, 324.

I.

Iceland spar, 399.

Illuminating power of gas, 774.

Illusion by movement of eye, 475. by persistence of vision, 476. stage, 290.

Images formed by lenses, 399, 616.

Impact theory of Stellar Evolution, 718.

Incandescent electric light, 528.

Incandescent gas-burners, 777.

INCLINED RAILWAYS, 125.

Inconstant, H.M.S., 152.

INDIA-RUBBER, 724.

INDIA-RUBBER AND GUTTA-PERCHA, 724.

Indicator, 9.

INDUCED CURRENTS, 502.

Induction coils, 503.

Injector, Giffard’s, 11.

Instantaneous photography, 623.

INTRODUCTION, 1.

IRON, 29.

IRON BRIDGES, 276.

IRON IN ARCHITECTURE, 72.

Iron, cast, 40. chemical changes of, 34. lighthouses, 596. meteoric, 32. ores, 39. pig, 40. ships, 133. smelting, 39. utility of, 30. wrought, 47.

Ismaïlia, 260.

J.

Jablochkoff’s electric candle, 525.

Jackson, 551.

Jacobi, 531.

Jamin’s magnet, 513.

Johannesburg, 694.

Joule, 804, 805.

Joy’s valve gear, 20.

Jupiter, 384.

K.

Kaleidoscope, 389.

Karoos, 705.

Kimberley, 705.

Kinetographic theatre, 479.

Kinetoscope, 478.

Kirchhoff, 422.

Klondyke, 692.

König or Kaiser Wilhelm, ironclad, 164.

König’s printing machine, 308.

Krupp’s guns, 214. steel, 55. works, 56.

L.

Lake Timsah, 260.

Lathe at Woolwich, 198.

Lathe, Blanchard, 96. screw-cutting, 87.

Lap of slide-valve, 9.

Lebel rifle, 188.

Lens, formation of image by, 616. in steps, 600. photographic, 616.

Lepidosiren, 685.

LETTERPRESS PRINTING, 306.

Leyden jar, 490. discharge, 538.

LIGHT, 380. electric, 497. invisible, 383.

LIGHTHOUSES, 593.

Limiting angle, 399.

Link motion, 16.

Linotype, the, 645.

Lithium, 425.

LITHOGRAPHY, 636.

Liverpool and Manchester Railway, 14.

Lock-gates, 267.

Locks in Manchester Ship Canal, 264, 266.

LOCOMOTIVE, THE, 14. balancing of, 20. compound, 18.

Lodge, Professor O., 540.

M.

MACHINE-GUNS, 218.

Madder, 796.

Magazine rifles, 187.

Magnesium, 720.

Magnetic field, 537.

Magnetism produced by current, 500.

Magneto-electric machines, 496, 507, 508.

MAGNETO-ELECTRICITY, 506.

Mallet’s Mortars, 212.

Malus, 405.

MANCHESTER SHIP CANAL, 262.

Manganese, 43.

Manhattan Life Insurance buildings, 78.

Mannlicher rifle, 187, 189.

Manufacturing v. making, 85.

Map, Channel Tunnel, 364. Manchester Ship Canal, 263. North Sea Canal, 271. Pacific Railway, 117. St. Gothard Railway, 372. Suez Canal, 256. Tower Bridge, etc., 299.

Marconi, 546.

Martini-Henry rifle, 184.

Mary Powell, the, 148.

Matter indestructible, 576.

Mauser rifle, 187, 188.

Maxim gun, 225.

Measuring machines, 88.

Mélinite, 748.

Menai Straits bridges, 280, 284.

Meteoric iron, 32.

Meteorites, 30.

Meteorology, importance of, 664.

Metereographs, 654.

Meters, gas, 775.

METROPOLITAN RAILWAY, THE, 114.

Microphone, 481.

Minié bullet, 180.

Mineral combustibles, 751.

Mines, submarine, 241.

Mirror, galvanometer, 570.

Mirrors, plane, 388.

Mirrors, illusions, 391–395.

Mitrailleur, 218.

Molecules, 733, 743.

Monarch, H.M.S., 156.

Moncrieff’s gun carriages, 208.

MONT CENIS TUNNEL, 351.

Montigny mitrailleur, 222.

Morse’s code, 560. instruments, 558. plate, 562. telegraphic line, 556. transmitting key, 561.

Mortar, Mallet’s, 212.

Mount Washington Inclined Railway, 125.

Musical sound, 666.

N.

Naphthaline, 669.

Napier’s platen machine, 321.

Nasmyth’s steam hammer, 26.

Nature knowledge, 1. printing, 640.

Nebulæ, 626.

Needle telegraphs, 553.

Negretti and Zambra’s recording thermometer, 659.

NEW METALS, 714.

New York, the, 148.

Newton’s prism experiment, 418.

Niagara Suspension Bridge, 287. Falls, 537.

NICARAGUA CANAL, 274.

Nichol’s prism, 403.

Niepce, J. N., 609. de Saint-Victor, 611, 615.

Nitro-benzol, 784.

Nitrogen and oxygen compounds, 732.

Nitro-glycerine, 734.

Nordenfelt gun, 223.

NORTH SEA CANAL, 271.

Note A—Production of coal, 812.

Note B—Conservation of energy, 812.

O.

Œrsted’s experiments, 548.

Oil springs, 760.

Oldbury, manufacture of aluminium, 722.

Ophthalmoscope, 423.

Optical apparatus of lighthouses, 598.

Orders of lighthouse apparatus, 602.

Organic bodies, 798.

Oscillating engines, 14

P.

PACIFIC RAILWAY, THE, 116.

Paddle-wheels, 130.

Page, Mr., 532.

PANAMA CANAL, 272.

Papier-maché stereotype process, 633.

PARAFFIN, 761. oils, 762, 763.

Parallel motion, 8.

Paris Exhibition, buildings of, 76.

Pascal’s principle, 325.

PATTERN PRINTING, 321.

Pepper, J. H., 392, 393, 505.

Percussion cap, 180.

PETROLEUM, 757.

Phenakistiscope, 476.

Phenomena of light, some, 382.

Phonautograph, 666.

PHONOGRAPH, 665.

Photographic camera, 615.

PHOTOGRAPHY, 607.

Photography, celestial, 636. in colours, 614, 630. in the dark, 613. X-Ray, 447.

Photolithography, 644.

Photozincography, 644.

Pig iron, 40.

Planes, Whitworth’s, 94.

Planets, photographs of, 636.

Planing machines, 92.

Plants in coal measures, 753.

Plaster of Paris, stereotype process, 633.

PNEUMATIC DISPATCH, 340. force, 333.

Pniel, 701.

Points, railway, 108.

Polariscope, 405.

Polarizer, 403.

Polytechnic institution, Regent Street, 505.

PORTABLE ENGINES, 24.

Portable telegraphic instruments, 556.

Portrait, Davy, 714. Helmholtz, 452. Joule, 789. Kirchhoff, 416. Lesseps, 249. Morse, 547. Senefelder, 632. Simpson, 731. Tesla, 572. Thomson, 481. Watt, 3. Whitworth, 85.

Port Saïd, 257.

Post-office railway van, 111.

Potassium, 715.

Powder, smokeless, 748.

Power, horse, 10. hydraulic, 324. of steam engine, 9. of locomotive, 21.

Powers, mechanical, 32.

Pressure gauge, 12. transmitted in fluids, 324.

Principle, the copying, 86. of the cantilever, 291.

PRINTING MACHINES, 305. processes, 632. telegraphs, 570.

Process blocks, 629.

Progress of mankind, 2.

Projectiles, 166. air’s resistance to, 175. deviation, 199. long range of Whitworth, 193. speed of, measured, 659. trajectory of, 174.

Propagation of sound, 668.

Prospecting, 361.

Proteus anguinus, 684. the modern, 807.

Pseudoscope, 472.

Puddling furnace, 45.

Q.

Queensferry, 292.

Quick-firing guns, 206. Hotchkiss, 208.

R.

RAILWAYS, 101. Great Western, 106. Metropolitan, 114. Midland, 112. London and Manchester, 102, 124. London and Woolwich, 103. Pacific, 116. St. Gothard, 371. Stockton and Darlington, 101.

Randt, the, 694.

Rangoon petroleum, 759.

Rays polarized, 401.

Réaumur’s steel, 67.

Recoil, 172.

RECORDING INSTRUMENTS, 653.

Red-short iron, 62.

REFLECTION OF LIGHT, 388.

Reflection in water, 396. total, 399.

REFRACTION, 397. double, 399.

Regulators for electric lamps, 523.

Reiss, 583.

Reliance building, the, 78.

Resistance, electrical, 494.

Resonance, electric, 540, 541.

Retina, 456.

Reverberatory furnace, 45.

Rifle, Brunswick, 180. Chassepot, 182. Lebel, 188. Lee, 188. Mannlicher, 187, 189. Martini-Henry, 184. magazine, 187. Mauser, 187, 188. military, 178. Minié, 181. Snider Enfield, 184. Vetterli, 189. Whitworth, 182.

RIFLED CANNON, 190.

Rifles, breech-loading, 182. reduction of bores of, 189.

Rifling, 171. guns, 199.

Righi, Professor, 541.

Rigi, railways ascending the, 126.

RIVER STEAMBOATS, 144.

ROCK BORING, 349.

“Rocket,” 14.

ROCK-DRILLING MACHINES, 355.

ROENTGEN’S X-RAYS, 445.

Rolling iron, 71.

Ronald’s telegraph, 548.

Roscoe, 425, 437, 444. and Bunsen, 664.

Royal Gun Factory, Woolwich, 27.

Ruete’s ophthalmoscope, 466.

Ruhmkorff’s coil, 730.

S.

Saint Paul building, the, 78.

Saltash Bridge, 283.

San Francisco, 123.

Sand, properties of, 253.

Saw, circular, 100.

SAWING MACHINES, 98.

Saturn, 440.

Schilling’s telegraph, 549.

Science, benefits of, 1.

Science and useful arts, 2.

Scott’s phonautograph, 669.

Screw, 86. cutting lathe, 87. dies and taps, 86. propeller, 116, 117.

Sea anemones, 679. horses, 684.

Secondary batteries, 530.

Segment shells, 217.

Senefelder, 556, 636.

Shear steel, 54.

SHIP CANALS, 249.

SHIPS OF WAR, 149.

Shrapnel shells, 216.

Siemens, 67.

Siemens’ dynamo, 522. pneumatic tubes, 341. regenerator, 68. regulator, 523.

Siemens-Martin steel, 70.

Sight, 452.

Signals, railway, 108.

Silver plating by electricity, 500.

Sirius, 440.

Skerryvore, 596.

Skiagraphs, 447.

Slide rest, 87.

Smokeless powder, 226.

Snider rifle, 184.

Snow-plough, 123.

Sodium, 715.

Some phenomena of light, 382.

Sommeiller perforators, 351.

Sömmering’s telegraph, 548.

Sound, 665. waves, 640.

Sounding telegraph, 566.

South African diamond fields, 701.

Speaking machine, 670. tubes, 730.

Spectra, absorption, 431. bright lined, 425. of permanent gases, 430. of salts, 424. of stars, 440. spark, 420.

SPECTROSCOPE, 416.

Spectrum, continuous, 422. lithium, 425, 436. of sodium, 424. pure, 420. solar, 418, 613.

Sphygmograph, 660.

Spiegeleisen, 64.

Stage illusions, 395.

Stars, distance of, 440. motion of, 442. spectra of, 440.

Steamboats, river, 144.

Steam engine, agricultural, 23. domestic, 25. forms of, 14. Newcomen’s, 3. portable, 23. Watt’s double-acting, 6. Watt’s improvements, 4.

STEAM ENGINES, 3.

Steam carriages, 21. expansive working of, 8, 17.

STEAM HAMMER, THE, 25.

STEAM NAVIGATION, 129.

Steam fire engine, 23. “navvies,” 269. rollers, 23. superheated, 9.

Steamships, comparative sizes of, 138. recent improvements in, 139. speed of, 137.

Steel, 52, 68. Bessemer, 58. blister, 54. cast, 54. Krupp’s, 55. puddled, 55. Réaumur’s, 67. shear, 54. Siemens, 68. tempering, 53. tensile strength of, 53.

Stellar evolution, 810.

Stephenson, George, 14, 102. Robert, 280.

ST. GOTHARD RAILWAY, 371.

Stereoscope, Brewster’s refracting, 470. Wheatstone’s reflecting, 469.

Stereoscopic effect, 469. lustre, 472. views, 470, 474.

STEREOTYPING, 642.

Stevenson, Alan, 602, 603.

Storage battery, 530.

Strada, 547.

Stratified discharge, 517.

Stroboscopic disc, 476.

Submarine cables, 575.

Sub-Wealden, exploration, 362.

SUEZ CANAL, THE, 251.

Sun, elements in, 438. constitution, 438.

Superheated steam, 9.

Surface plates, Whitworth’s, 94.

SUSPENSION BRIDGES, 284.

Sympathetic needles, 547.

Syphon recorder, 571.

Swan’s carbon process, 619, 481.

T.

Table, air resistance to projectile, 177. benzol and toluol compounds, 789. coal raised in Great Britain, 727. coal-tar, colours, 794. composition of cast iron, 43. dimensions of parts of eye, 462. dimensions of steamships, 138. electric light v. gas, 514, 515. floatation of Monarch and Captain, 160. formulæ of hydro-carbon, 758. Gramme machine, 518. hydro-carbon in coal-tar, 782. illuminating power of gases, 774. lighthouse apparatus, 602. lighthouse lamps, 597. Martini-Henry rifle, 187. nitrogen and oxygen compounds, 731, 732. photographic actions, 612. products from 100 lbs. of coal, 796. ships of war, 166. telegraph code, Morse’s, 560. telegraph, war department, 557. tenacities of iron, 277. wave-lengths of colours, 411. Wheatstone’s dot signals, 565.

Talbot, 610.

Talbotype, 611.

TALL BUILDINGS, 76.

Tawell, arrest of, 550.

TELEGRAPHIC INSTRUMENTS, 553.

TELEGRAPHIC LINES, 572.

Telegraph poles, 572.

Telegraphs in Great Britain, 574.

Telegraphy, wireless, 546.

Tel-el-Kebir, 260.

Telepherage, 549.

TELEPHONE, THE, 581.

Telestereoscope, 472.

Tenacities of iron, 277.

Tension of electricity, 498.

Tesla, Nikola, 542. oscillator, 542.

Tesla’s experiments, 545.

Thallium, 426.

The Terrible, H.M.S., 167.

Thomson, Sir W., 483, 570, 571, 805, 808.

Throttle valve, 6.

Thunderer, H.M.S., 164.

“Times” newspaper, 387, 312.

TOOLS, 85.

Torpedo-boats, 231.

TORPEDOES, 227.

Tour de Cordouan, 593.

Tourmaline, 404.

TOWER BRIDGE, THE, 297.

Trajectory of projectile, 174.

Tramways, 22.

Transfer process, 638.

Transvaal, 693.

Tunnel, Mont Cenis, 351. St. Gothard, 373.

Tunnels, helicoidal, 379.

Turbine, 144.

Turbinia, the, 144.

Turret ships, 154.

Tyndall, 383, 803, 809.

U.

Undulation of the ether, 409.

V.

Vaal River, 701.

Valentine, electric, 581.

Valve, throttle, 6. slide, 8.

Vegetable nature of coal, 753.

VELOCITY OF LIGHT, 384.

Venice, view in, 396.

Vetterli rifle, 189.

Victoria, Australia, gold-field of, 688.

Victoria Bridge, Montreal, 282.

Victoria, H.M.S., 166, 168.

Vision, persistence of, 476.

VISUAL IMPRESSIONS, 468.

Voltaic element, 490.

Vulcanite, 728.

Vulcanized india-rubber, 727.

W.

Wall papers, machines for printing, 322.

Walter press, 323.

Warner’s torpedo experiment, 241.

Warren girder, 280.

Warrior, H.M.S., 150.

Warships, new types of, 167.

Water decomposed by electricity, 498.

Waterproof cloths, 726.

Watt, 3, 4, 8, 14.

Welding, electric, 537.

Wells, Horace, 735.

Welsbach incandescent gas lights, 777.

Whaleback, 147.

Wheatstone, 387, 470, 474, 552. and Cooke’s telegraph, 548, 554.

Wheatstone’s automatic telegraph, 564. dial telegraph, 568. dot signals, 565.

Wheels of railway carriages, 107.

Whitehead’s torpedoes, 232, 242.

Whitworth’s guns, 193. rifle, 182.

Wire-drawing for Atlantic cables, 576.

Wireless telegraphy, 546.

Woodbury printing process, 641. process for engraving photographs, 643.

Woolwich, 27. “infants,” 201.

Work, 11, 12, 160, 325, 804.

X.

X-RAYS, 445.

Y.

Young’s paraffin oil, 762.

Z.

Zinc, amalgamated, 490.

Zincography, 644.

Zoetrope, 478.

Zöllner, 475.

EDINBURGH: PRINTED BY MORRISON AND GIBB LIMITED

------------------------------------------------------------------------

TRANSCRIBER’S NOTES

1. Added “near Bristol” after “146. Clifton Suspension Bridge,” on p. xii. 2. Added “PLATE IX.” to caption of illustration facing p. 144. 3. Added “PLATE XI.” to caption of illustration facing p. 168. 4. Switched the captions for “Fig. 99.” facing p. 210 with the caption for “Fig. 102” facing p. 206. The captions were out of order and did not agree with the content of the illustration. 5. Changed “a calibre of 4,724 in.” to “a calibre of 4·724 in.” on p. #208. Otherwise the muzzle opening would be larger than its length. Confirmed online. 6. Changed “the two rows in” to “the two rows is” on p. 332. 7. Added “O T” after “P P, or” on p. 398. 8. Added “PLATE XVII.” to caption of illustration facing p. 422. 9. Changed “screw motion to finally” to “screw motion to finely” on p. 433. 10. Changed “which strangely makes it” to “which strangely makes its” on p. 491. 11. Changed “Duke of Wellington’s statute” to “Duke of Wellington’s statue” on p. 550. 12. Changed “-” to “·-” for the Morse Code for A in the table on p. 565 to agree with the text that follows. 13. Changed “vegetation which spontaneously makes it” to “vegetation which spontaneously makes its” on p. 678. 14. Added “INDEX” header on p. 813. 15. Silently corrected typographical errors. 16. Retained anachronistic and non-standard spellings as printed. 17. Enclosed italics font in underscores. 18. Superscripts are denoted by a carat before a single superscript character or a series of superscripted characters enclosed in curly braces, e.g. M^r. or M^{ister}. 19. Subscripts are denoted by an underscore before a series of subscripted characters enclosed in curly braces, e.g. H_{2}O.

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