Chemistry and its borderland

Front matter

SIR WILLIAM CROOKES, O.M., LL.D.,F.R.S

Med

Chemistry and its Borderland. By Alfred W. Stewart, D.Sc. London, New York, Bombay, and Calcutta : Longmans, Green, and Co. 1914. (C . /V- *•<•-• if- This book gives a very interesting account of some recent developments of chemistry, such as can readily be followed by the general reader who is not altogether ignorant of the elements of the science. The relations of chemistry, firstly to the other sciences and then to the industries, are treated in the first chapters, and then recent advances in some special branches which have developed more or less independently are admirably summarised. Thus a chapter is devoted to immuno-chemistry and another to spectroscopic work. Radio-activity and the nature and the transmutation of the elements are considered at some length, and the last chapters are given to the discussion of some chemical problems of the present and the future and to the methods and organisation of chemical research. These later chapters contain some excellent suggestions for schemes for training research chemists, and throughout the book great stress is laid upon the practical and commercial value of pure research. The author writes in a style which is neither too technical nor too elementary, and he is particularly happy in his choice of illustrations and analogies to explain some rather difficult conceptions.

Preface

At the present time, the sciences and their applications are playing a preponderant part in our daily existence , and yet, so involved are their ramifications that very few people realize the closeness of contact between science and modern life. An interest in the progress of scientific investigation is becoming more marked, however, and a demand is springing up for information upon recent advances in the scientific field.

This demand is easy to supply in certain branches of science ; for some lines of scientific thought lend themselves more readily than others to exposition in non-technical language. It is comparatively easy for a biologist or a physicist to give an account of the root principles of his subject such as can readily be understood by the general reader, though the latter may remain ignorant of the minutiae of the evidence upon which the principles are founded. Chemistry, on the other hand, does not lend itself so well to popular treatment ; for this science has evolved a symbolical method of expression of its own which, while to the expert it is more lucid than words, is a stumbling-block to the ordinary man who has not been trained to grasp it. Deprived of its aid, the chemist feels hampered in exposition ; and, as a consequence, chemical works designed for the general public are apt to err either in being too elementary in treatment or in taking too much knowledge for granted on the part of the reader.

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PREFACE

In the present volume, it is hoped that these errors have been avoided. No chemical symbols have been employed, and purely elementary questions have not been dealt with at length ; so that the non-technical reader will find in the following pages an account of some recent developments in chemistry couched in language which need

not present any impediment.

The book may roughly be divided into four sections. In the first of these (Chapters I-III) an attempt has been made to show how chemistry has internally split into various subdivisions, while simultaneously it has expanded into other fields, and has lent the aid of its methods to other sciences and to industry. The second section (Chapters IV-VII) contains chapters on chemical subjects each of which has developed on more or less independent lines. The next section (Chapters VIII-XII) centres round radium and the various problems which it suggests as to the structure of matter. The final section (Chapters XIII-XV) is intended to give some insight into the methods and organization of modern research work, both on the technical and the human side— a field practically unknown to those who are not directly connected with the scientific world.

In conclusion, it may be well to draw attention o another point : the practical and commercial value of pure research. It is very difficult for the non-scientific reader to realize the fact that all the applications of science which render our modern life more comfortable have taken their rise, not in the hands of the applied scientist, but in the laboratories of investigators who were striving after knowledge for its own sake. To take only a single examp e, the electric lamps of our cities, the electric trains on our railways, the magnetos of our motors and aeroplanes are merely developments of a single experiment ma e y Faraday in 1831 for a purely scientific purpose. This

PREFACE

IX experiment resulted in the discovery of the principle of the electric motor and dynamo ; but at the same time when the principle was brought to light no one could have foreseen the vast influence which the new discovery was to exert upon the conditions of social life. Work of the same kind is being carried out in our laboratories to-day, and in the future much of it will no doubt take its place along with Faraday’s experiment ; but in this country the general public is rather apt to demand immediate dividends and to scoff at the value of pure research work. In the present volume, therefore, it has been thought well to draw attention to a few instances in modern chemical research where investigations which at first sight seemed to be of merely academic interest, have laid the foundations of farextending industries providing work for thousands of men. These examples might have been multiplied ; but the ones which have been selected are sufficient to indicate that money spent on pure research is not by any means wasted, even from the narrowest commercial point of view.

My thanks are due to Dr. H. T. Clarke and Dr. J. E. Macllwaine, who have assisted me by suggestion and criticism ; to Sir William S. McCormick, for supplying me with the statistics of the Carnegie Trust for the Universities of Scotland ; to the Secretary to the Royal Commissioners for the Exhibition of 1851, for furnishing me with a copy of the latest Report of the Commissioners ; and to Miss F. W. Rea, B.A., B.Sc., and Mr. A. K. Macbeth, M.A., B.Sc., for reading the proofs of the volume.

A. W. S.

The Queen’s University of Belfast, January , 1914.

Contents

PAGE

CHAPTER

I. The Ramification of Chemistry 1

Chapter I

When the sciences of the present day are passed in review, the most striking feature which appears to characterize them all is the process of division and subdivision through which they pass. Sciences which, fifty years ago, were treated as unities are now being broken up into sections and subsections, each of which is acquiring a kind of semi-independent status ; so that the original science is becoming a mere convenient grouping of the various younger branches. Thus geology has become split up into stratigraphy, which deals with the various formations occurring on the earth’s surface ; petrology and mineralogy, which are concerned with rocks and their structure ; and palaeontology, whose subject is fossils. Physics has long been partitioned into a similar series of departments : statics and dynamics, which treat of bodies at rest and in motion ; magnetism and electricity ; sound, light, and heat ; and, in quite recent times, radioactivity. Chemistry has been no exception to this rule : time and again in its history it has thrown out an offshoot which has developed into a semi-independent science ; and in this chapter an attempt will be made to indicate the

B different fields which are covered by these subdivisions of the subject.

At the beginning of last century chemical science was approaching a stage of fission, the first of many. A very considerable amount of exact information had been accumulated with regard to the properties and preparation of substances, but the major portion of these data referred to the compounds of metals or to mineral products. Substances such as oil of cloves and turpentine had been investigated, but no means of preparing them artificially had been devised ; and it was taken for granted that these vegetable products could only be formed by the intervention of vital action. Animal extracts were also supposed to come into the same category ; so that at this period chemistry was divided into two sections : inorganic chemistry, which dealt with materials of a metallic or mineral type, such as iron or lime ; and organic chemistry, under which head were included all substances whose formation appeared to be due to vital action.

In 1828, however, this sharp division was broken down by a research of Wohler on a substance called ammonium cyanate. This body he had prepared by purely “ inorganic ” methods ; but on evaporating its aqueous solution, he found, to his astonishment, that it became transformed into a new body, which he was able to identify as urea. Now, urea is a constituent of human urine, and up to that time it had been regarded as one of the most typical “ organic ” compounds, the production of which depended on vital action. Yet Wohler’s researches had shown that it could be prepared from purely inorganic materials without any direct vital intervention. This discovery at once swept away the arbitrary division between inorganic and so-called organic derivatives, with the result that chemistry became again

unified. , . , ,

This unity, however, was not destined to rema unbroken. The next decades proved that one element stood out above all others in the number of its derivatives : carbon compounds were found to exceed in number all the derivatives of the other elements taken together ; and it became clear that a division of chemistry into carbon and non-carbon derivatives would be necessary, both on account of the number of carbon compounds, and also because the properties of all carbon derivatives had much in common, while their differences from the compounds of other elements were strongly marked. An examination of the list of carbon compounds showed that practically all of them lay within the limits of the discarded definition of organic chemistiy, as they were for the most part derived from plants or animals ; so that it seemed simplest, instead of inventing a new name, to revive the old one of organic chemistry, and include in this field every compound of carbon. At a later stage we shall trace the scope of organic chemistry, but first let us glance at the older branch : inorganic, or non-carbon chemistry.

Inorganic chemistry, as -we have said, now includes the chemistry of all the compounds which do not contain any carbon. It will be seen that this field is a very varied one, for it comprises all the metallic elements as well as such substances as phosphorus, nitrogen, sulphur, and chlorine. A rough classification of these bodies is required, and the usual one is into metals and non-metals ; but this is an arbitrary rather than a correct division, for there are some substances, such as arsenic, which have the general properties of metals, but which, in addition, show other properties resembling those of some non-metallic substances. These intermediate elements are termed metalloids.

Some idea of the problems involved in inorganic chemistry may be obtained if it be assumed that a new element has been found, and that its nature has to be investigated. In the first place, the element must be traced to its source, and the chemist must discover where it occurs most plentifully, so as to be sure of a sufficient supply of material for his further investigations. His next step would be to prepare as many compounds of it as he could, and from analyses of these he will be able to draw certain conclusions as to the relationship of the new element with other elements already known. For example, he may find that it forms a chloride which contains one atom of the new element combined with one atom of chlorine. Now, the analyses of the chlorides of other elements show that one atom of sodium combines with one atom of chlorine ; one atom of magnesium unites with two atoms of chlorine ; whereas an atom of aluminium can attract three atoms of chlorine. It is therefore safe to say that the new element is more akin to sodium than it

is to either magnesium or aluminium.

At this point we must be on our guard against a possible mistake. For example, although charcoal and diamond are totally different in their physical properties yet investigation shows that one atom of the diamond element combines with four atoms of chlorine ; and the same is true of the element which forms charcoal. Further examination of the two chlorine derivatives shows them to be identical ; this proves that both the diamond and the charcoal are built up from the same element, which turns out to be carbon. Thus carbon is able to exist under various guises-charcoal, diamond, and graphite; yet at first sight i these three forms appear to be totally different. This phenomenon is termed allotropy, and when an element appears in more than one physical modification, it is said to exist in allotropic modifications. Thus diamond, graphite, and charcoal are allotropic forms of the one element carbon. Red and yellow phosphorus are examples of the

same phenomenon.

To ensure that our hypothetical new element is not a simple allotropic modification of some already known element, a series of compounds of the new body must be prepared, and these must be compared with corresponding compounds of the known elements ; for the phenomenon of allotropy is confined to the element, and does not persist when the element is transformed into a compound.

Having thus established the individuality of the new body, attention would be turned to determining its physical constants, such as melting-point, specific heat, atomic weight, and so forth ; and from the data furnished by all this work an endeavour might be made to allocate a place to the element in the Periodic Arrangement of the elements. The methods by which this can be done will be dealt with in a later chapter,1 so that we need not enter into them here.

This brief account will give some idea of the processes requisite in the study of inorganic chemistry. Of course, in the case of elements and compounds of commercial importance, further lines of investigation suggest themselves ; for methods which are successful in laboratory experiments on a small scale are not always applicable when a process has to be carried out with tons of material instead of ounces. A by-product which is of little account in a laboratory investigation may represent the difference between a paying and a non-paying commercial process ; so that in chemical industry we are working under limitations which have to be very carefully investigated before a process can safely be launched as a commercial speculation.

Let us now turn to organic chemistry, which was the first group to segregate itself from the main line of general chemistry. As we have seen, this division between the two allied subjects arose on account of the extraordinary number of carbon derivatives which were discovered ; and in order to explain this wonderful preponderance of carbon among compounds we must look more closely into the matter.

1 See Chapter VIII.

Non-carbon compounds, as a general rule, contain comparatively few atoms in their molecules, and these atoms are usually of different elements. It is not often that we find many atoms of the same element in an inorganic body. For example, in water there are two atoms of hydrogen and one of oxygen ; in Sulphuric acid two atoms of hydrogen are united with one of sulphur and four of oxygen , in sulphuretted hydrogen two atoms of hydrogen are joined to one of sulphur. Compare this with one of the compounds of carbon and hydrogen which contains no less than sixty carbon atoms united with a hundred and twenty-two hydrogen atoms. The explanation of this difference between the carbon and non-carbon derivatives is to be found in the fact that of all the elements carbon has the greatest power of forming what are called chains of atoms. We can join together in a chain no less that sixty carbon atoms, and this chain will unite with a hundred and twenty-two hydrogen atoms. No other element appears to possess this power in anything approaching this degree, though nitrogen atoms can be joined in short chains of two or three atoms under certain conditions. Thus the carbon atom differs from all other elementary atoms in its power of combining directly with other carbon atoms.

When this was recognized, and when, the methods oi forming such chains had been worked out in the laboratory, it became a simple matter to produce very complicated carbon derivatives ; and the result has been an enormous outpouring of synthetic material. At the present day, considerably more than a hundred thousand different com pounds of carbon are known, and their number is emg increased almost daily. Even in the early days of the science the burden of this vast array of impounds made itself felt ; for the author of a large text-book on organic chemistry published in the early part of last century is sal to have suggested ironically that the organic chemists of that day might take a rest for a few years, as otherwise he despaired of ever making his book complete. At the present time it is almost impossible to keep in touch with the flood of publications which comes out in the various journals , and the Chemical Society publishes monthly a series of abstracts of the most important papers which have appeared, so as to save its members from wading through the details of the originals. Some idea of the bulk of the material may be gathered when it is realized that these condensed abstracts, each of which runs, usually, to merely a few lines, amounted in 1912 to over a thousand pages. Even the Society s “ Annual Report on Organic Chemistry,” which is supposed to include only the chief work of the year, extends to over a

hundred pages of small type.

No one can fail to see that with such a mass of material in our hands we are forced to divide it as far as possible into sections. How can we do this in a logical manner ? The problem is really simple. Carbon compounds may be separated into two classes. In the one class, the compound forms a straight chain, each atom following another like links. If, now, the two ends of the chain be joined, a ring will be formed ; and certain carbon compounds are known which do actually possess a structure of this type. Thus the whole of the carbon compounds can be classified into two sets : open-chain compounds and cyclic compounds. Then the latter class is divided into two subsections ; for the ring may be made up of carbon atoms only, or it may be partly carbon atoms and partly other elements which go to form it. If it is entirely made up of carbon atoms joined together in a circle, then the compound is termed a homocyclic one ; whereas if we have in the ring say four carbon and two sulphur atoms, the compound is called a heterocyclic one. Examples of the open-chain type are to be found among the fats and sugars ; turpentine, camphor, and such-like aromatic products are examples of homocyclic bodies ; while the heterocyclic series includes alkaloids like quinine, morphine, and caffeine, as well as more complex substances like the colouring matter of the blood.

The problems before the organic chemist are complex in the extreme, much more so than is the case with inorganic bodies. The subject is a very technical one, and it is almost impossible to give any simple examples of the methods employed ; so that we must confine our attention to the most general types of problems.

Since most of the natural products which are used in commerce are derivatives of carbon, it is natural that one of the main features of organic chemical research should be the attempt to discover the way in which these bodies are built up, with a view to producing them synthetically in the laboratory. Let us see if we can gain some insight into the methods employed, while avoiding as far as possible any technical details.

A rough analogy will help us to some extent. Imagine that we were given a watch, and that we were asked to make a replica of it. What would our procedure be ? In the first place, we should examine the external part of the watch and take measurements of the case ; then we should open the watch and endeavour to learn how it worked. After this, before we could proceed further, we should have to take the machine to pieces and discover what wheels it contained and how they were related to each other. Finally, we should build an exactly similar watch from the data we had acquired. Now the organic chemist, when faced with a new compound, attacks the problem in a somewhat similar manner. In the first place, he submits his crude substance to a laborious process of purification in order to make sure that he is dealing with a single compound and not a mixture of two or more bodies. He then investigates the physical properties of his pure sample ; which would correspond to our measurements of the watch-case. Next, he tries various reagents upon the substance, with a view to finding out its chemical behaviour as a whole ; this would be parallel to our examination of the working of the watch. He then applies different reactions to the compound and endeavours to break it down into simpler substances which he can recognize more easily than the original complex body. This corresponds to our taking the watch to pieces wheel by wheel. From a study of the various decomposition products which he has obtained, the chemist tentatively draws up a plan of the structure of the original compound, and in this formula he expresses by means of symbols all the reactions which the body can exhibit. Finally, in order to be perfectly certain that his plan of the compound’s structure is the correct one, he must proceed to synthesize a substance having the structure which he has assigned to the original body. If, when it is prepared, this synthetic substance has all the properties of the natural compound, his task is complete.

This short summary may give a false impression of ease, for, when expressed like this, the task seems an absurdly simple one ; but the experimental difficulties in the way are often enormous. Reactions do not always produce good returns of the required material, and much time may have to be spent in finding a way round a difficulty. Another line of attack may have to be chosen after months of labour have been expended ; and very often researches in one branch of the subject may have to wait until further light is thrown upon compounds which at first sight appear to have little connection with the point under examination. An example of such a series of researches is to be found in the history of synthetic indigo. The first steps in this work were taken in 1841, but it was not until 1882 that a simple method of synthesizing this body was fully developed ; while eight years more were spent in evolving a process suitable to work

IO on a manufacturing scale. The results of this research have, however, been momentous ; for at the present day sj nthetic indigo is being put on the market at such a low price that the natural Indian product is practically out of the competition. Thus a steady half-century’s pure research has transferred one of the great industries from India to Germany.

To give some idea of the way in which organic chemistry is interlaced with our modern life, we may take a single example : the chemistry of coal-tar. It would probably puzzle the ordinary non-chemical person to give a list of ten substances obtained directly or indirectly from the distillation of coal ; but it would assuredly prove a much more formidable problem if we asked an organic chemist to give a complete list of these products, for their number is almost beyond reckoning. Let us deal with the main outlines of the coal-tar industry, refraining from detailed description.

In the early days of the trade, the only products sought for on a large scale were illuminating gas and coke. The former is very largely made up of carbon compounds which are chemically allied to paraffia wax, though the physical properties of the two substances do not in any way suggest such a close relationship. As a by-product in the coal distillation, however, great quantities of tar are produced ; an for many years no peculiar use was found for this materia apart from its employment as a preservative of wood and so forth. About the middle of last century, however, the study of coal-tar was undertaken by various chemists ; and from their researches a perfect maze of industries has sprung up all over the world. In fact, at the present day the value of coal-tar far exceeds that of the illuminating gas which is produced at the same time ; and we might almost say that what was originally a mere by-product ha now sprung into the chief place in the industry, while

THE RAMIFICATION OF CHEMISTRY u illuminating gas, once the main object of the distillation, has sunk into a secondary position.

The products of the coal-tar industry are so numerous that no detailed description of them can be attempted here. We must confine ourselves to the history of a single development, and pass over the others with the briefest mention. Before the middle of the last century practically all the dyes in common use were natural products, derived from plants for the most part. In 1856, however, Sir W. H. Perkin, then a boy of eighteen, was testing a process by which he hoped to produce synthetic quinine ; and in the course of his work he accidentally obtained a body which seemed worthy of further investigation. His subsequent researches showed that he had produced a synthetic dye, aniline mauve, the first of many similar products which are manufactured at the present day. Now, at the base of this dye lay a substance called benzene ; and as the researches upon coal-tar which were then going on had shown that benzene was one of the tar-constituents, the way was opened for a commercial utilization of the by-products of the gas-works. Young Perkin started a factory at Greenford Green for the manufacture of aniline mauve and several similar dyes which, he had discovered, could be prepared by analogous methods. In the history of British technical chemistry the succeeding period is one which must always be looked on with regret. Britain was the country which not only had the greatest supplies of coal-tar, but was also • the greatest producer of this material ; she had great dyeworks, and everything seemed to point to the development of a new industry in which she was not only better equipped than her possible competitors, but in which she actually had a start in the race. It will hardly be credited that such a chance was allowed to slip, but the short-sightedness of her manufacturers was such that this actually occurred. No attempts were made to set the new development going on a large scale. Germany stepped into the breach, captured the industry, and has since profited both directly and indirectly from her far-sightedness. For not only did she gain control of the synthetic dyes, but the trained personnel engaged in this industry developed other lines of work, with the result that at the present day she has practically the whole trade of organic technical chemistry in her hands. When we read the following words, in a report on the Exhibition of 1862 made by Hofmann, who was Sir W. H. Perkin’s teacher, we cannot but feel that a chance was put before our manufacturers in the plainest manner, though their contempt for scientific chemistry at that time prevented them from appreciating it : —

“ England will, beyond question, at no distant day, become herself the greatest colour-producing country in the world; nay, by the strangest of revolutions, she may, ere long, send her coalderived blues to indigo-growing India, her tar-distilled crimsons to cochineal-producing Mexico, and her fossil substitutes for quercitron and safflower to China, Japan, and other countries whence these articles are now derived.”

This revolution has taken place, but it is not England which exports these colouring matters. Within forty years of the first discovery of the coal-tar dyes, Germany’s export of these substances had risen to a value of £4,500,000 annually, while ten years later, in 1907, this output had been practically doubled.

Leaving this rather melancholy subject, let us turn o the other products which are obtained in the c°al'ta^ industry, and here we come into touch with many different fields. Medical science utilizes the coal-tar derivatives which organic chemistry has prepared; drugs such as antipyrine, aspirin, saccharin, and antifebrm , antisep tcs like carbolic acid and salol ; hypnotics such as hypnone. Bacteriology employs the coal-tar colours to s am 1 s microbes so as to render them easier to examine. Photography utilizes developers most of which are synthesized from substances found in coal-tar. The perfume industry relies greatly upon the same source ; while military science is indebted to organic chemistry and coal-tar for explosives such as lyddite. Future developments may show that in the organic products of coal-distillation we have a source of some hydrocarbon which will help to eke out the supply of petrol for our motors ; benzole, which is at present utilized to some extent, is a by-product in gas-manufacture. Many other examples might be quoted ; but we have said enough to show how closely the coal-tar industry has become interwoven with our daily life, and how organic chemistry, in the brief space of half a century, has sprung into a commanding position in the community.