Text Book of Physical Geography

Front matter

DR CLYDE'S SCHOOL GEOGRAPHIES, Revised according to the Treaty of Berlin, Now ready, 19th Edition, price 4s. School Geography. By James Clyde, M.A/, LL.D., lately one of the Classical Masters in the Edinburgh Academy. Revised throughout and largely Re-written. " His valuable and interesting compendium." — Athenæum. " A book of great excellence. . . . Altogether, it is a first-rate school book."— School Guardian. "Dr Clyde's manual comes nearer to what, in our opinion, a school geography should be, than any book which has yet been published.' — School Magazine. "We have no doubt this is one of the very best school geographies: it stands in the first class, and has but few competitors. Dr Clyde has thoroughly succeeded in making his book interesting. . . . It is by far the fullest and best arranged we have seen." — National Schoolmaster.

"The grand characteristic of the f School Geography ' is its singular readahleness — its clear, fluent, lively narrative; the sunny ray of realistic art that everywhere brightens the subject with the charm almost of romance, dispelling the heavy cloud of superfluous facts and figures. Its author has apparently a strong sense of the picturesque aspect of geography, which shows itself in a peculiar vividness of description, an artistic handling of local colour, a happy knack of singling out salient points, and an academic nicety of literary touch. ... In conclusion, we can heartily recommend tbe ' School Geography' as a model work of the kind, no less on account of its scholarship and b_auty of style, than of its perfect typography." — Educatio?tal News. Also, Revised according to the Treaty of Berlin.

Clyde's Elementary Geography. 19th Edition. Price Is. 6d. "A thoroughly trustworthy manual." — Educational Times. DR CLYDE'S GREEK SYNTAX. Fifth Edition, largely Re-written, price 4s. 6d. Greek Syntax ; With a Rationale of the Constructions. By Jas. Clyde, M.A., LL.D. With Prefatory Notice by John S. Blackie, Professor of Greek in the University of Edinburgh ; and a Summary for the use of Learners and a chapter on Accents. 4s. 6d. V A Specimen Copy of o_ or more Books sent to Principals of Schools, post-free, on receipt of half-price in stamps, by Oliver and Boyd. Edinburgh: Oliver asd Boyd. London: Simpkin, Marshall, and Co SPALDING'S ENGLISH LITERATURE, Revised and Continued. 14th Edition. History of English Literature : With an Outline of the Origin and Growth of the English Lan guage. Illustrated by Extracts.

Preface

TO THE FIRST EDITION. This Text-Book of Physical Geography is intended chiefly as a Manual for Students preparing for the South Kensington Examinations and for Senior Classes. Long experience as a teacher has convinced me that there is a want of some work on Physical Geography which, while embracing the whole range of the subject, shall at the same time be suitable, both with respect to style and price, to science classes. Several excellent text-books are already in existence, but in some cases the price is such as to put them beyond the reach of students, while very often the technical character of the style is far beyond all but the most intelligent of them. A complete manual, in simple language and at a moderate price, would therefore seem to be a desideratum. The example of Professor Tyndall, whose profound researches are detailed in language of singular clearness, has en couraged me, though longo intervallo, in the attempt to combine simplicity of style with scientific accuracy. How far I have succeeded remains to be seen. A work of this kind must of necessity partake somewhat of a compilation ; but in the grouping of

PREFACE.

. facts, and in some of the deductions drawn from them, I may lay claim to some originality. A series of Examination Papers is given at the end, which the reader will no doubt find useful ; but I have not con fined myself to the scope of these papers. My aim. has been to give a complete view of the whole range of Physical Geography as fully and as connectedly as the size of the book would allow. I have endeavoured to consult those authorities which carry most weight in their several departments ; and in the selection of facts have given the preference to those of the most recent date. It would be tedious to mention all the sources from which information has been gained, and a list of chief authorities would look like unnecessary display. In almost every instance, however, they are mentioned in the text.

W. L. St Mark's College, Chelsea, January 1874. I.— The Crust op the Earth. Page Form and Size of the Earth; Its Position in space. Gravitation; the Earth's Density ; Interior of the Earth. Chemical Elements ; Chemical Combination ; Oxygen ; Oxides ; Silicon. Metals ; Potassium ; Sodium ; Calcium ; Magnesium ; Aluminium. Chemical Affinity. Heat — its Eelation to Motion ; Conduction and Convection. Electricity ; Electric Conductors. Magnetism ; the Inclination and Dip of the Keedle. The Earth a Magnet, 9-20

Chapter II

— Geological Systems. District Strata observable in Eocks ; Origin of Stratified Rocks ; Fossils; Duration of Geological Periods; Connexion between Geology and Physical Geography. Classification of Rocks; Aqueous and Igneous; Metamorphic Socks; Plutonic Rocks. Geological Systems; the Laurentian, Cambrian, Silurian, Devonian, Carboniferous, Permian, Triassic, Oolitic, Cretaceous, and Tertiary Systems. The Glacial Period, 21-35

Chapter III

— Distribution of Land — Islands. Changes still going on in the Natural World ; Post-Tertiary Period ; No Great Changes since Man's Appearance on the Earth; Distribution of Land ; Theories connected with it. Comparison of tha Northern and Southern Continents in Form and Contour. Classification of Islands. Arctic Lands ; Last Voyage of Franklin ; Greenland; Its Extent and Character. Antarctic Lands; Discoveries by Sir James Ross. Coral Islands; The Coral Polype; Coral Formations. Fringing Reefs, Barrier Reefs, Encircling Reefs, Atolls; Theory respecting these Formations. Areas of Depression 36-47 IV.— Earthquakes. Page Origin of the Force which has Upheaved the Continents ; Connexion with Earthquakes and Volcanoes. Earthquakes of Peru in 1868. Cause of Earthquakes ; Depth of Focus ; Earth-waves ; Classificacation of Earthquakes ; Periodicity of these Movements. Earthquake of Lisbon ; and of Calabria. Geographical Distribution of Earthquakes. Remarks on their Destructive Character, . 48-57

Chapter V

— Volcanoes. Connexion between Earthquakes and Volcanoes. Eruption of Vesuvius in 1872. Geographical Distribution of Volcanoes. Volcanic Products ; Mud- Volcanoes. Cause of the Explosion which produces Earthquakes and Volcanoes. Theory of Subterranean Heat; Probable connexion with Electro Magnetism; Mr Mallet's Theory, 58-68

Chapter VI

— Denudation— Great Plains. What is meant by Denudation; Different Effects of Marine and Aerial Denudation. Formation of Hills and Mountains. Effect of Glacier Action ; The Effects produced by Rivers ; Formation of Caiions and Deltas. Floods, their Use; Deserts not a Permanent Feature in the Geography of the Globe. Classification of Plains into Sandy Deserts, Grassy Plains, and Forest Plains, 69-81

Chapter VII

— Europe and Asia. Surface of Europe : The Pyrenees, Mountains of Spain and France ; The Alps, the Apennines, the Balkans, the Carpathians, Plateaux of Bohemia and Bavaria ; Great Plain of Europe. The Scandinavian Mountains, the Ural Mountains, the Caucasus. Surface of Asia ; Plateaux of Armenia and Asia Minor ; Peninsula of Arabia ; Plateaux of Iran and Pamir. The Himalayas. Great Plain of Hindostan; The Deccan. Plateau of Thibet; The Kuen-lun Mountains; The Karakoram Range. Eastern Turkestan. The Thian Shan Mountains ; The Altai System. Great Central Plateaux of Asia ; Desert of Gobi. Manchuria ; China Proper ; Further India. The Japan Archipelago; Borneo; Sumatra, 82-96

Chapter VIII

— North and South America. The Rocky Mountains : The Cascade Range; The Sierra Nevada. Passes in the Rocky Mountains ; Pacific Railway. Great Central Plain of North America. The Alleghany Mountains ; The Atlantic Slope. Surface of Mexico : Central America ; The West India CONTENTS. 5 Pago Islands. Surface of South America; The Andes; The Panine" System; Mountains of Brazil ; El Gran Chaco; Patagonia, . 97-110

Chapter IX

— Africa and Australia. Surface of Africa : The Atlas Mountains ; The Sahara ; Region of the Nile and Niger; Discoveries of Du Chaillu; Abyssinia; Region of the Congo ; Discoveries of Livingstone ; Region of the Zambesi ; Kalahari Desert ; Cape Colony ; Madagascar. Surface of Australia : The Dividing Range ; History of Discovery ; Tasmania; New Guinea; New Zealand, 111-124

Chapter X

— The Atmosphere and its Uses. Extent of the Atmosphere: Its Pressure; Composition; Ozone. Light ; Theory respecting it ; Reflection and Refraction ; Cause of Twilight; Mirage. Theory of Colour; The Rainbow. Spectrum Analysis. Nature of Heat ; Effect of Aqueous Vapour in the Atmosphere ; Effect of Elevation upon Temperature. Sound, how caused ; Velocity. Electricity ; Magnetism ; Connexion with the Sun's Spots, 125-138

CHAPTER XL

— Winds and Storms. Daily Variations of the Barometer. Land and Sea Breezes ; Trade Winds; Return Winds; Calm Belts; The Trades cross at the Equatorial Calm Belt ; Movement of the Calm Belts ; Monsoons ; Variable Winds. Cyclones, their Nature, Direction, Force, and Time of Occurrence. Account of the " Alabama." Rotation of Variable Winds 139-151

Chapter XII

— Clouds and Rain. Evaporation; Clouds; Elevation of Clouds; Never Motionless, though apparently so ; Colour of Clouds. Fogs ; Dew ; Rain. Cause of Rain ; Its connexion with the Prevailing Winds ; Abundance in the Tropics. Distribution of Rain ; Rainless Districts. Long Droughts of the Tropics. Comparison between the Northern and Southern Hemispheres with respect to the amount of Rain, 152-163

Chapter XIII

— Snow and Glaciers. Formation of Snow; the Snow-line; Snow Limit; Hail. Formation of Ice ; Ground Ice. Glaciers, how formed ; Cause of Motion ; Regelation; Effects of Glacier Action; Moraines. Glaciers of Mont Blanc; Great Aletsch Glacier ; Glaciers in the Himalayas ; In New Zealand. Origin of Icebergs ; Effect on Climate, . 164-1 XIV.— Springs, Rivers, and Lakes. Page Origin of Springs : Classification ; Artesian Wells ; Intermittent Springs ; Thermal Springs ; Geysers ; Mineral Springs. Sources of Rivers; Flood Moderators; Great Oceanic River Basins; Inland Basins ; The River Oxus. Lakes, their Classification ; Salt Lakes. Origin of Lake Basins. Lakes of Central Africa; Lake Titicaca ; Lake Torrens 175-188

Chapter XV

— Rivers and Lakes of Europe. Watershed of Europe: The Neva, Vistula, Oder, Elbe, the Rhine; Falls of Schaffhausen; Falls of the Handeck ; Staubbach ; Lakes of Switzerland ; Scenery of the Middle Rhine ; Delta of the Rhine. The Volga, Dnieper, Danube ; Source of the Danube ; The Iron Gate ; Delta. The Po ; Lakes of Italy ; Delta of the Po. The Rhone; Lake Geneva; Delta. Lakes of Scandinavia, . 189-204

CHAPTER XVL

— Rivers and Lakes of Asia. The Obi ; Yenesei ; Lake Baikal ; The Lena ; Fossil Ivory. The Amoor ; Advance of Russia on the East Coast ; The Hoang-ho ; Change in its Bed; Tang-tse-Kiang ; Lake Poyang; Different Names of the River. The Mekong, Irawaddy, Brahmapootra ; The Ganges; Delta; The Indus; The Euphrates and Tigris, 205 218

Chapter XVII

— Rivers and Lakes of North America. River Systems of North America: The Mackenzie; The Saskatchewan ; St Lawrence ; Great Lakes of North America— Superior, Huron, Michigan, Erie, and Ontario; Falls of Niagara. The Mississippi ; The Missouri ; Delta ; Floods. Rio Grande ; Streams draining the Atlantic Slope. The Yukon ; The Fraser ; The Columbia; The Colorado; Canons, 219-232 CHAP. XVIII. — Rivers and Lakes of the Southern Continents. The Orinoco; The Amazon, its Source, Tributaries, Width, Tides, Floods, Navigation ; The Rio de la Plata ; Its Estuary. Rivers of Africa: The Niger; Mungo Park; The Delta. The Nile; Its Source; Victoria Nyanza, Albert Nyanza, Tanganyika; Cataracts of the Nile, annual rise ; The Congo ; The Lualaba ; The Zambesi. Rivers of Australia ; The Murray 233-251

Chapter XIX

- The Oceans and Currents. The Pacific, Atlantic, and Indian Oceans ; Depth of the Oceans; Methods of Sounding ; Telegraphic Plateau ; Voyage ofthe " dial- CONTENTS. 7 lenger ;" Bed os the Atlantic ; Bed of the Indian Ocean. Pressure of the Water; Colour; Salts; Temperature. Oceanic Currents, their cause ; The Gibraltar Current ; Dr Carpenter's Theory of the Circulation ; Description of Oceanic Currents ; The Gulf Stream ; Cold Currents ofthe Atlantic; Gulf Stream of the Pacific; Open PolarSea; Voyageof the "Polaris." Waves, their cause ; Ground Swell, ... .... 252-269 CHAPTEE XX.— Climate and Weather. Definition of Climate; Zones of Temperature ; Cause which Influence Climate — Nature of the Surface, Presence of Water, Currents of the Ocean ; Isothermal Lines ; Isotheral and Isothermal Curves ; Classification of Climates ; Northern Hemisphere warmer than the Southern ; Climate of the British Islands. Difference between Climate and" Weather; Indications of Change in the Weather, ... 270-283

Chapter XXI

— Minerals. Coal : Origin of Coal-measures ; Coal-fields of Great Britain ; of the United States ; Anthracite, Plumbago, and Jet. Iron : Quantity raised in Great Britain; Magnetic Iron; Meteoric Iron. Mineral Veins ; Connexion with Magnetic Currents and with Metamorphic Rocks. Copper; Tin; Lead. Gold: Discovery of Gold-fields during the Present Century; Largest Gold Nugget. Silver : Quantity raised in England ; Silver Mines of Chili, Peru, Mexico, Nevada. Quicksilver; Platinum. Gems and Precious Stones: The Diamond; Ruby; Sapphire; Emerald; Beryl; Opal; Garnet; Topaz; Turquoise; Cat's-Eye ; Lapis Lazuli; Agate; Onyx. Salt: Salt Mines of Wielicza, 284-296

CHAPTER XXIL

— Vegetation. Vegetation : Food of Plants ; Universal Distribution ; Division into Acotyledons, Monocotyledons, and Dicotyledons ; Proportion of these in Tropics, Temperate, and Arctic Regions. Vegetable Zones. Origin of Vegetation. Flora of the British Islands; Arctic Flora; Influence of the Direct Rays of the Sun. Temperate Regions of Europe and Asia ; The Mediterranean Region; The Arabian Region ; Indian Flora; Flora of Eastern Asia ; The Indian Archipelago; African Flora; Madagascar; North America; Central America; Tropical America; The Forests of the Amazon; South America; Australia; New Zealand; Oceanica. Marine Vegetation 297-316 XXIIL— Animals. Page General Distribution of Animals ; Migrations ; Origin of Present Distribution. Zoological Regions. Distribution of Different Classes of Animals : Mammalia, including Quadrumana, Marsupials, Edentata, Pachydermata, Carnivora, Rodentia, Ruminantia, and Cetacea; Birds, including Rapaces, Passeres, Scansores, Columbæ, Gallinæ, Cursores, Grallatores, and Natatores ; Reptiles, including Testudines, Sauria, Ophidia, and Batrachia.

Fishes : best Fisheries in Cold Currents; Migrations; Colour; Phosphorescence. Radiata; Corals; Growth of the Red Coral; Foraminifera; Lowest Form of Animal Life. Insects : Most Numerous in the Nevf World ; Ants of Trinidad ; Driver- Ants. Microscopic Insects ; Use of Insects in the Economy of Nature, .... 317-336

CHAPTER XXIV.

- The Human Race. Varieties in the Human Race ; Antiquity of Man ; Classification according to Languages. The Mongolidæ, Atlantidæ, Iapetidæ ; Subdivisions of these Families. Influence of Man upon Nature ; Climate ; Domestication of Plants and Animals ; Acclimatization ; Colonization ; Ascendancy of the White Races ; Conclusion, 337-345 Examination Papers, 346-375 Index, , 377-386 TEXT-BOOK OF PHYSICAL GEOGRAPHY.

Chapter I

The Crust of the Earth. The earth on which we live is a large ball or globe isolated in space. The circumference of this globe is nearly 25,000 miles, and its diameter about 7912 miles. It derives its light and heat from the sun, which is about one million times larger ; and the mean distance of the earth from the sun is 91,350,000 miles. The earth makes an annual revolution round the sun in a path called its orbit ; while at the same time it spins round on its axis, making one revolution every day. Several other globes besides the earth revolve round the sun, some larger than the earth, some less ; some nearer to the sun, others more remote. All these bodies, the earth included, are called planets. In very early times astronomers distinguished between the planets or "wanderers" and fixed stars.

The former shine by the reflected light of the sun, just as the moon does, or as the earth would appear to do if viewed from the moon, or from one of the neighbouring planets. The stars shine by their own light like the sun itself, and indeed are each of them suns. The nearest of the fixed stars (a Centauri) is at least 200,000 times further distant than the sun, while others are at a much greater distance. Each star is probably the centre of a system of planets similar to our solar system. The earth's motion round the sun is the result of two forces acting in combination. Centrifugal force and the force of Gravitation. Centrifugal force acting alone, would carry the earth away into space, but gravitation keeps it revolving round the sun. An illustration of these forces is seen when a boy uses a sling.

The two strings which he holds in his hand act like the force of gravitation, and keep the stone whirling round his head. When one string is let go, the stone is set free, and away it flies owing to the centrifugal force which it has attained by its rapid motion round and round. According to the law of gravitation, bodies attract each other in proportion to the mass of matter which they contain, and to their proxi mity to each other • or more precisely, every atom of matter in the universe attracts every other with a force which varies inversely with the square of the distance between them. Thus the earth attracts the moon, and keeps it in its orbit, but the moon also attracts the earth, and forms the tides. Every object upon the earth's sur face is drawn towards its centre, and the weight of a body which we feel in lifting it, is but a measure of this attrac tion.

Some bodies are heavier than others of the same volume, simply because they contain a greater mass, or, as we might say, a larger number of atoms. As each separate atom exerts the force of gravity, substances in which the atoms are allowed to move freely, naturally assume a globular form. We see this exemplified on a small scale by the dew drops which hang on the blades of grass ; and it is owing to the same law that the planets are all more or less perfect spheres. A knowledge of the law of gravitation actually enables us to ascertain the weight of the earth. The calculation depends upon the principle of comparing the attraction of the earth for a small body with that exerted on the same body by some other mass of definite size and weight. In the year 1772, the mountain called Schehallion, in Perththe crust or the earth. 11 shire, was selected as suitable for this purpose.

A plumb line was suspended in tbe neighbourhood of this mountain, and it was noticed that the line was slightly deflected from the perpendicular, owing to the attraction of the mountain. The amount of deflection was carefully measured, as was also the size of the mountain. It was then calculated what the amount of attraction would be in a mass the same size as the earth, and composed of materials similar to those of which the mountain was composed. The result arrived at was that the earth as a whole had a mucli greater density than the mountain mass. In other words, while granite has a density of about two and a half times that of water, the mean density of the earth was calculated to be about double that of granite. Since then, other and more accurate methods have been adopted to test the earth's density, and it is now generally admitted that it amounts to about five and a half times that of water.

An opinion at one time prevailed that the interior of the earth was a molten mass, and that this internal fire was surrounded by a comparatively thin crust of solid land. Mathematicians have proved that this opinion is no longer tenable, and it would be hard to reconcile it with the great. density of the globe, Even supposing the interior of the earth to be solid granite, we could not account for this density, and it has been conjectured that it consists for the most part of metals, most probably iron. Confining our attention, however, to that portion of the earth's crust which comes within the limits of human observation, we find that it is composed of a vast number of substances of various kinds, and of different combinations. But just as the thousands of words which compose the English language are built up out of the letters contained in the alphabet, so the various substances found in the crust of the globe are composed of a comparatively few elements.

Of these, sixty-six are at present known to the chemist. Some half-dozen have been discovered within the last twenty years by means of spectrum analysis, but they do not seem to play any very important part in the economy of nature. Of the sixty-six Chemical Elements, fifty-one are metals, leaving only fifteen non-metallic substances. All the metals, except mercury, are solids; mercury and a non-metallic element called bromine are, in ordinary temperatures, liquids ; while other elements, such as oxygen and hydrogen, are gases. Mercury, when exposed to the cold of the polar regions, becomes solid, and may be ham mered like lead ; on the other hand, when exposed to a high temperature, it is changed into a vapour or gas. Chlorine, which under ordinary circumstances is a gas, can be condensed by extreme cold and pressure into a liquid ; oxygen, hydrogen, and nitrogen have also recently been liquefied by a similar process, and hydrogen even solidified.

All the metals are easily fusible, though at different temperatures, and if the neat be increased, pass from fluids into gases. Some substances, however, such as carbon, never liquefy, but with a high temperature pass at once from the solid to the gaseous state. The simple elements are very unequally distributed in nature. Thus oxygen forms one-fourth part by weight of the whole atmosphere, and nearly nine-tenths by weight of the whole of the water on the globe. It also forms, in combination, nearly one-half of the solid crust of the earth. Silicon forms about a quarter of the earth's crust ; next rank in order, aluminium, iron, calcium, magnesium, sodium, potassium, and carbon. These nine together have been estimated to constitute TVc_i of the earth's crust, the other elements together contributing less than one-fortieth.

The atmosphere is mainly composed of two gases, nitrogen and oxygen ; water is composed of oxygen and hydrogen. But in one case the gases are merely mixed; in the other they are chemically combined. Chemical com bination is said to take place when the compound body loses the characteristics of its constituents, and forms an entirely new substance. Thus water has no resemblance either to oxygen or hydrogen, one of which is tbe most in flammatory, and the other the most inflammable of gases. In the air, however, there is merely mechanical mixture of two gases without any chemical change. As oxygen is THE CRUST OF THE EARTH. 13 the heavier gas of the two, it was at one time thought that in the upper regions of the air we should meet with an excess of nitrogen. It was found, however, by experiment, that this was not the case, and the circumstance is explained by what is termed the law of diffusion.

This law, which holds among all gases, is to the effect that the particles of one gas have a tendency to interpenetrate those of another, though of different weight, so that the two are equally diffused. Chemical compounds have been arranged into three divisions : binary compounds, or those containing but two elements, as water, for example ; ternary compounds, con sisting of three elements, such as marble, which consists of carbon, oxygen, and calcium ; and quaternary compounds, which contain four elements, as alum, which consists of sulphur, oxygen, aluminium, and potassium. The chief binary compounds are those of oxygen with some other substance. Such compounds are termed oxides ; thus iron rust is the oxide of iron ; red lead is the oxide of lead ; platina, soda, magnesia, and silica are the oxides respec tively of platinum, sodium, magnesium, and silicon.

The compounds of chlorine are termed chlorides ; thus common salt is the chloride of sodium. Oxides which contain a large proportion of oxygen exhibit acid properties : sulphuric acid is a compound of oxygen and sulphur ; carbonic acid of oxygen and carbon. Other binary com pounds besides oxides are acids : thus, hydrochloric acid is a compound of hydrogen and chlorine. When an oxygen acid combines with certain other oxides, such as soda and magnesia, a salt is produced. The name of a salt is formed by changing the termination of the acid, and placing it before the name of the oxide. Thus sulphuric acid combined with soda forms the sulphate of soda, or Glauber's salt ; with magnesia it forms the sulphate of magnesia, or Epsom salt. These salts are of course ternary compounds. The oxides which, when united with an acid, form salts, are termed basic oxides, or more briefly, bases.

Silicon does not exist in a natural state, but its oxide, B TIIYSICAL GEOGRAPHY. silica or silicic acid, is the chief constituent in the solid crust of the earth. Sand is generally composed of quartz grains, and quartz is almost pure silica. Sandstone is simply consolidated sand, the particles having been com bined by pressure, or by the infiltration of lime, oxide of iron, or other cementing material. Granite is a compound of quartz, felspar, and mica. Felspar is chiefly composed of silica, alumina, and potash ; while mica is a compound of silica, potash, and magnesia. Flint is an impure form of silica. Gneiss is also almost entirely composed of it ; while a multitude of other materials out of which rocks are formed, such as trap, basalt, lava, and clay are silicates — that is, compounds of silicic acid with a metallic oxide.

The metals which compose such a large proportion of the simple elementary substances have been divided into four classes : 1. Metals of the alkalis ; 2. Metals of the alkaline earths ; 3. Metals of the earths proper ; 4. Metals proper. To the first group belong potassium and sodium. The term alkali was first applied by the Arabians to the carbonate of soda found in the ashes of seaweed, and afterwards to the carbonate of potash obtained by burning land plants. These carbonates are now termed mild alkalis, in contradistinction to the caustic alkalis, soda and potash, obtained from them. The characteristic properties of an alkali are a peculiar taste, the power of changing vegetable reds into blues or greens, and yellows into browns ; and the power of neutralizing aeids when added in sufficient quantities.

Thus soda and potash neutralize carbonic aeid when combined with it. Potash and soda, it will be remembered, are not simple elements, but oxides of the metals potassium and sodium respectively. Potassium has a brilliant white colour shaded with blue, and in ordinary temperatures is soft like wax. At the freezing point, however, it is hard and crystalline. As a metal it has no particular use. The substance is found chiefly as potash, in granite, trap, and other igneous rocks, and is associated with silica. From the disintegration of these rocks it finds its way into soils, from whence it is extracted THE CRUST OP THE EARTH. by plants. Sodium, like potassium, is never found free, and can only be obtained by difficult chemical processes. It has a reddish-white colour, and is soft and adhesive at ordinary temperatures.

As a compound it is very abundant in most soils, and in marine plants. The chloride of sodium is common salt. The second group of metals comprises calcium and magnesium, and two others, barium and strontium, less widely distributed. All the metals of this class are obtained with difficulty, and are never found but in combination with oxygen or some other element. Calcium is described as a silver-white metal. Combined with oxygen it forms quicklime. The carbonate of lime is limestone ; and the sulphate, gypsum. Chalk is a softer kind of limestone, while marble is a hard crystalline form of it. The finer varieties of gypsum form alabaster and selenite. Mag nesium is a metal said to resemble zinc. Its oxide, magnesia, readily combines with carbonic acid, and is sometimes found in nature as a carbonate.

Combined with the carbonate of lime it forms magnesian limestone. Among the metals of the earths proper, aluminium is the most important. It is never found in a native state, but can be produced at a moderate expense, and has been put to a variety of uses. Its oxide, alumina, is very abundant in nature. Combined with silica it forms clays of various kinds ; and the silicate of alumina combined with potash forms felspar, one of the constituents of granite and other igneous rocks. The disintegration of granite is owing to the action of rain-water charged with carbonic acid. The acid dissolves the potash and a portion of the silica, leaving the excess of silica and the alumina still united. The sulphate of alumina combined with the sulphate of potash forms alum. In the group of metals proper are included gold, silver, platinum, iron, lead, copper, quicksilver, nickel, tin, zinc, antimony, arsenic, manganese, and bismuth, and others less common.

Gold, silver, platinum, and a few others are occasionally found in their natural state as metals. The great majority, however, are chiefly found as ores. Sometimes the ores are alloys or combinations of different metals. More frequently they are salts or oxides of the metal. The ores of lead, zinc, copper, and antimony are frequently sulphurets. Platinum is the heaviest metal known, and is nearly twice the weight of lead. Carbon forms the greater part of the structure of all organic substances, and it is remarkable for the number of its compounds It exists in nature in a pure state in the diamond. Next to the diamond, the purest natural form of carbon is graphite or plumbago, commonly called black lead. Iron is frequently present in graphite, but in its purest form it consists entirely of carbon.