All substances, whether solid, liquid, or aëriform, are supposed to consist of hard separate atoms or particles, and in conformity with that supposition to be surrounded by the ethereal medium, otherwise they could not transmit light and heat, which are merely vibrations of that medium. Even the hardest and most compact substances are capable of compression, and have been compressed to an enormous degree by the hydraulic press; but it probably transcends mechanical force to bring their atoms into contact: in fact, no known substance is impervious to _both_ light and heat, however thin. 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.