hardness and durability eminently fit it for the important part it plays in binding the materials of rocks together, and enabling them better to resist the decomposing effects of air and water.

Carbon, though found in a nearly pure state in the clear gem called diamond, and also in the black opaque mineral, graphite, more usually occurs mixed with various impurities, as in the different kinds of coal. This element has a high importance in nature, because it is the fundamental substance made use of by both plants and animals to build up their structures, and because it serves as a bond of connection between the organic and the inorganic worlds. In union with oxygen, carbon forms the widely-diffused gaseous compound known as carbon-dioxide (CO2), which occurs in the proportion of about four parts in every ten thousand parts of ordinary atmospheric air. From the air it is abstracted and decomposed by living plants in presence of sunshine, the oxygen being in great measure sent back into the atmosphere, while the carbon with some oxygen, nitrogen, and hydrogen is built up into the various vegetable cells and tissues. When we look at a verdant landscape or a boundless forest, it is a striking thought that all this vegetation has been chiefly constructed out of the small proportion of invisible carbon-dioxide present in the atmosphere. The vast numbers of beds of coal imbedded in the earth's crust have, in like manner, been derived from the atmosphere through the agency of former tribes of plants. Not only in beds of coal, but still more prevalently in masses of limestone, carbon enters into the composition of rocks. Carbon-dioxide, as was pointed out in chapter ii., is abstracted by rain in passing from the clouds to the earth, and is also supplied by decomposing plants and animals in the soil. It is readily dissolved in water, and forms with it carbonic acid, CO(OH)2, which has been referred to as so powerful a solvent of the substance of many rocks. This acid unites with a number of alkaline and earthy bases to form the important family of Carbonates. Of these the most abundant is calcium-carbonate, or carbonate of lime (CaCO3), which consists of 44 per cent carbon-dioxide, and 56 per cent lime. This carbonate not only occurs abundantly diffused through many rocks, but in the form of limestone builds up by itself thick mountainous masses of rock many hundreds of square miles in extent. It is abstracted by plants to form calcareous tufa (chapter v.), but far more abundantly by animals, especially by the invertebrata, as exemplified by the familiar urchins, corallines, and shells of the sea-shore. The limestones of the earth's crust appear to have been mainly formed of the calcareous remains of animals. Hence we perceive that the two forms in which carbon has been most abundantly stored up in the earth's crust have been principally due to the action of organised life; coal being chiefly carbon that has been taken out of the atmosphere by plants, and limestone consisting of carbon-dioxide, to the extent of nearly one-half, which has been secreted from water by the agency of animals.

Sulphur is found in the free state, more particularly at volcanic vents, in pale yellow crystals or in shapeless masses and grains; but it chiefly occurs in combination. Some of its compounds are widely diffused among plants and animals. The blackening of a silver spoon by a boiled egg is an illustration of this diffusion, for it arises from the union of the sulphur in the egg with the metal. Combinations with a metal (sulphides) and combinations with a metal and oxygen (sulphates) are the conditions in which sulphur chiefly exists. Hydrogen is a gas which has been detected in the free

state at active volcanic vents; but otherwise it occurs chiefly in combination with oxygen as the oxide water (H2O), of which it constitutes about one-ninth, or 11.12 per cent, by weight. It also enters into the composition of plant and animal substances, and forms with carbon the important group of bodies known as Hydrocarbons, of which mineral oil and coal-gas are examples. In smaller quantity, it is found united with sulphur (sulphuretted hydrogen, H2S), with chlorine (hydrochloric acid, HCl), and a few other elements.

Chlorine is a transparent gas of a greenish-yellow colour, but except possibly at active volcanic vents it does not occur in the free state; united with the alkali metals, potassium, sodium, and magnesium, it forms the chief salts of sea-water. The most important of these salts, sodium-chloride, or common salt (NaCl) contains 60.64 per cent of chlorine, and forms 2.64 per cent by weight of sea-water. This salt is found diffused in microscopic particles in the air, especially near the sea, and beds of it hundreds of feet thick occur in many parts of the world among the sedimentary rocks that constitute most of the dry land.

Phosphorus does not occur free; it has so strong an affinity for oxygen that it rapidly oxidises on exposure to the air, and even melts and takes fire. Its most frequent combination is with oxygen and calcium, as calcium-phosphate or phosphate of lime (Ca3(PO4)2, p. 182). Though for the most part present in minute proportions, it is widely diffused in nature. It occurs in fresh and sea water, in soil and in plants, especially in their fruits and seeds; it is supplied by plants to animals for the formation of bones, which when burnt are found to consist almost entirely of phosphate of lime.

Fluorine also is never met with uncombined; it never unites with oxygen, forming in this respect the sole exception among the elements; its most frequent combination as a rock constituent is with calcium, when it forms the mineral Fluor-spar (CaF2). Like phosphorus, it is widely diffused in minute proportions in the waters of some springs, rivers, and the sea, and in the bones of animals.

To these metalloids we may add the colourless, tasteless gas Nitrogen, which, though not largely present in the earth's crust, constitutes four-fifths by volume or 77 per cent by weight of the atmosphere. It does not enter into combination so readily as the other elements above enumerated, but it is always found in the composition of plants and is a constituent of many animal tissues. It is the principal ingredient of the substance called ammonia, which is produced when moist organic matter is decomposed in the air. In many rocks composed wholly or in great part of organic remains, such, for instance, as peat and coal, nitrogen is a constant constituent.

METALS. Though so large a proportion of the known terrestrial elements are metals, these are much less abundant in the earth's crust than the metalloids. The most frequent are Aluminium, Calcium, and Magnesium. The substances most familiar to us as metals occupy an altogether subordinate part among rocks, the most abundant of them being Iron.

Aluminium never occurs in the free state, but can be artificially separated from its compounds when it is seen to be a white, light, malleable metal. It is almost always united with oxygen as the oxide of Alumina (Al2O3) which occurs crystallised as the ruby and sapphire, but is for the most part united with silica, and in this form constitutes the basis of

M

the great family of minerals known as the Silicates of Alumina, or Aluminous Silicates. These silicates generally contain some other ingredient which is more liable to decomposition, and when they decay and their more soluble parts are removed, they pass into clay, which consists chiefly of hydrated silicate of alumina.

Calcium is not met with uncombined, but has been artificially isolated and found to be a light, yellowish metal, between gold and lead in hardness. It occurs in nature chiefly combined with carbonic acid as a carbonate, and with sulphuric acid as a sulphate, to both of which substances reference has already been made; it is also present in many silicates. So abundant is calcium-carbonate or carbonate of lime in nature that it may be detected in most natural waters, which dissolve it and carry it in solution into the sea. Its presence in rocks may be detected by a drop of any mineral acid, when the liberated carbon-dioxide escapes as a gas with brisk effervescence. Calcium-sulphate is likewise a common constituent of terrestrial waters, especially of those which in household management are called hard; it constitutes not less than 3.6 per cent of the salts in ordinary sea-water, and when sea-water is evaporated this sulphate (gypsum), being least soluble, is the first to be precipitated in minute crystals resembling in shape those shown in Fig. 62.

Magnesium is likewise only isolated artificially, when it appears as a soft, silver-white, malleable and ductile metal. It occurs in sea-water combined with chlorine as magnesiumchloride, which constitutes 10.8 per cent of the total proportion of salts. It unites with carbonic acid as a carbonate, which with carbonate of lime forms the widely diffused rock called magnesian limestone or dolomite; it also enters into