noring the intermediate liquid) a comparison of the gaseous with the solid state of matter at once shows that the former is, not the end, but the beginning of the evolution. The gas is not only comparatively indeterminate—without fixity of volume, without crystalline or other structure, etc.—but it also exhibits, in its functional manifestations, that simplicity and regularity which is characteristic of all types or primary forms. Looking, first, to the purely physical aspect of a gas—I speak, of course, only of gases which are approximately perfect, to the exclusion of vapors at low temperatures and of gases which are readily coercible: its volume expands and contracts inversely as the pressure to which it is subjected; its velocity of diffusion is inversely proportional to the square root of its density; its rate of expansion is uniform for equal increments of temperature; its specific heat is the same at all temperatures, and, in a given weight, for all densities and under all pressures; the specific heats of equal volumes of simple and incondensible gases, as well as of compound gases formed without condensation, are the same for all gases of whatever nature, and so on. In all these respects the contrast with both the liquid and solid forms, the relations of whose volumes, or structures, or both, to temperature and to mechanical pressure or other force are complicated in the extreme, is great and striking. But this contrast becomes still more signal, secondly, under the chemical aspect. We cannot, in any proper sense, assign the proportions of volume in which the combination of solids and liquids takes place—indeed, the combination of solids as such is impossible—and the numbers expressive of the proportions of the combining weights upon their face exhibit an appearance of inflation and irregularity which the most sustained endeavors of scientific men (such as Dumas, Strecker, Cooke, L. Meyer, Mendelejeff, and Baumhauer) have been unable to obliterate. In the combination of gases, on the contrary, all is simplicity and order. "The ratio of volumes, in which gases combine, is always simple, and the volume of the resulting gaseous product bears a simple ratio to the volumes of its constituents"—such is the law of the combination of gaseous volumes known as the law of Gay-Lussac. By weight, the ratio of combination between hydrogen and chlorine is 1 to 35.5; by volumes, one volume of hydrogen combines with one volume of chlorine (the volumes being taken, of course, at the same pressures and temperatures) so as to form two volumes of hydrochloric acid. Oxygen and hydrogen combine in the proportion of 16 to 2 by weight; but one volume of oxygen combines with two volumes of hydrogen, forming two volumes of watery vapor. Nitrogen and hydrogen, whose atomic weights, so called, are 14 and 1 respectively, combine in the simple ratio of one volume of nitrogen to three volumes of hydrogen, the combination resulting in two volumes of gaseous ammonia. And carbon, whose 'atomic weight' is 12, though it cannot be actually obtained in gaseous form, is assumed by all chemists (for reasons not necessary to
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THE POPULAR SCIENCE MONTHLY.