220 THE POPULAR SCIENCE MONTHLY. 



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 ex- 

 treme, 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 irre- 

 lation and irregularity which the most sustained endeavors of scien- 

 tific 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 hy- 

 drogen 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 com- 

 bine in the proportion of 16 to 2 by weight ; but one volume of oxy- 

 gen 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 re- 

 sulting in two volumes of gaseous ammonia. And carbon, whose 

 'atomic weight' is 12, though it cannot be actually obtained in gas- 

 eous form, is assumed by all chemists (for reasons not necessary to 



