By far the greater number of terrestrial substances consist of heterogeneous atoms chemically combined into atomic systems or molecules; but there are sixty-four which have never yielded to chemical analysis, and are therefore believed to be respectively formed of only one kind of atoms. Thirty-five of these are metals found either pure or as ores, and sixteen are metals existing naturally in chemical combination with alkalies, alkaline earths, or earthy bases, that is as salts, from which they have been obtained by the analytical power of electricity or other means. The thirteen remaining simple substances are non-metallic: some are aëriform, some solid, one liquid.
The alkaline metals are sodium, potassium, lithium, cæsium, rubidium, and thallium. They are distinguished by their energetic affinities for, and the simplicity of their compounds with, non-metallic elements. They are never met with native, and are amongst the most difficult metals to reduce from their ores, and their spectra are remarkable for simplicity. Sodium and potassium—which have been such important agents in spectrum science—were reduced from their alkalies of soda and potash by Sir Humphry Davy by means of the voltaic battery, a discovery which led the way to the reduction of many of the others. Lithium is a white metal which burns brilliantly in air and oxygen; it swims in naphtha, and is the lightest solid body known. Cæsium is the most energetic of all metals in its chemical affinities.
The metals of the alkaline earths are barium, strontium, calcium, and magnesium. They possess, like the preceding, energetic affinities for the non-metallic elements, and are reduced with difficulty from their ores. Barium is obtained from earth baryta: it is powerfully alkaline, and its salts are colourless and poisonous. Calcium is obtained from limestone, chalk, marble, and gypsum, which are amongst the most abundant constituents in the crust of the earth; it is a bright ductile metal of a bronze colour. Magnesium, which is a brilliant silver-white hard brittle metal, is obtained from magnesium limestone or dolomite. Although the ores of calcium and magnesium cover vast areas of the globe, the metals form a very small comparative proportion of them.
The metals derived from non-alkaline earths are glucinum, yttrium, thorinum, zirconium, and aluminium, which is the only one of any interest: it is now becoming a very useful metal. It combines readily with oxygen to form clay. The ruby, sapphire, and oriental topaz are merely coloured varieties of corundum, which is nothing but crystallised clay. Rubidium, cæsium, and thallium were discovered by spectrum analysis.
The avidity of some of these metals for oxygen is quite remarkable: potassium and rubidium inflame when they touch ice or cold water; they decompose the water and combine with its oxygen. Calcium becomes luminous in warm water, and burns with intense light when heated to redness; but a magnesium wire burns with such intense brilliancy that it has been employed for photography, and will probably become useful for household purposes, as two ounces and a half of magnesium wire when burnt give a light equal to that of twenty pounds’ weight of stearine candles.
The metals whose oxides are not reducible by heat without the aid of some form of carbon include nearly all the useful metals. They are all polyatomic, that is, they combine with other elements in the number of atoms varying from two to eight, and are divided into seven groups in regard to this property. For instance, zinc, copper, and cadmium are diatomic. Zinc is invaluable as a source of electric light and heat in the voltaic battery, and its vapour burns brilliantly. Copper is one of the most useful of metals, while cadmium is of no value at all. Nickel, cobalt, and uranium form the triatomic group; they are remarkable for their complex spectra. Nickel is usually an ingredient in meteorites; cobalt is employed in pigments and in sympathetic inks; and the oxide of uranium is used to stain glass, and gives it some very peculiar properties, as will be shown. The precious metals have a feeble affinity for oxygen at any temperature, and their oxides are decomposed by heat alone, and sometimes even by the undulations of light.
Metals are excellent conductors of heat, but they vary exceedingly in that respect; both theory and experiment prove that the best conductors are invariably the worst radiators. In fact those atoms which transfer the greatest amount of motion to the ethereal medium, that is, which radiate most powerfully, are the least competent to communicate motion to each other, that is, to conduct with facility. Silver and copper are the best conductors of heat, but the worst radiators. These two metals are the best conductors of electricity, but it is influenced by temperature; for MM. Matthiessen and Von Bose’s experiments have proved that all pure metals in a solid state vary in conducting power to the same extent between zero and 100° Cent., and that the alkaline metals conduct electricity better when heated than when cold.
All metals are capable of being vaporized, but at very different degrees of temperature. Platinum requires the heat of the oxy-hydrogen blowpipe, which by estimation amounts to 8801° Cent. This property makes it valuable for terminal points to the conducting wires of the voltaic battery and magneto-electric induction machine where great heat can be employed without fusing the platinum terminals. Copper is always employed for the conducting wire on account of its superior conductive power. The coil of wire in the magneto-electric machine, which is often miles long, is insulated by a coating generally of green silk thread. But in experiments of extreme delicacy where magnetism might vitiate the results, perfectly pure copper wire which is diamagnetic is used for the conducting wires in the thermo-electric pile of the goniometer, and the wires are coated with white silk thread, since it was discovered that the green dye contains some magnetic metal.
The mass of the metals however constitutes comparatively but a small part of the terrestrial globe, which is formed of chemical combinations of only thirteen simple elementary substances,—a wonderful manifestation of creative power that could form a world of such variety and beauty by means of atoms so little diversified; still more wonderful is it that four simple elements alone constitute the basis of nearly the whole organic fabric. The air we breathe, water, the bodies of men and living creatures, and the vegetation that adorns the earth, are chiefly combinations of three invisible gases, oxygen, hydrogen and nitrogen, with carbon, the purest amorphous form of coal.
Oxygen gas forms three-fourths of the superficial crust of the terrestrial globe, its productions and its inhabitants. At least a third part of the solid crust of the earth is oxygen in combination; it constitutes eight parts out of nine in water, and water covers three-fourths of the surface of the globe; it forms more than twenty parts out of a hundred of atmospheric air, and in the organic kingdom it is an essential constituent. Except in the atmosphere, oxygen is never uncombined, but may be obtained by distilling chlorate of potash, by the decomposition of water by voltaic electricity, and by other means. When pure it is a colourless, tasteless, inodorous, invisible gas; it is incombustible at ordinary temperatures, yet absolutely essential to combustion; no animal can live long in it, and none can exist without it. In the atmosphere oxygen is highly magnetic; its magnetism increases with cold and decreases with heat; hence its intensity varies with night and day, winter and summer, but its magnetic property vanishes when it enters into composition.