Page:Scientific Papers of Josiah Willard Gibbs.djvu/408

 relation between temperature, pressure, and volume, for the vapor of each of these substances differs widely from that expressed by the usual laws for the gaseous state, the laws known most widely by the names of Mariotte, Gay-Lussac, and Avogadro. The density of each vapor, in the sense in which the term is usually employed in chemical treatises, i.e., its density taken relatively to air of the same temperature and pressure, has not a constant value, but varies nearly in the ratio of one to two. And these variations are exhibited at pressures not exceeding that of the atmosphere and at temperatures comprised between zero and 200 or 300 of the centigrade scale.

Such anomalies have been explained by the supposition that the vapor consists of a mixture of two or three different kinds of gas or vapor, which have different densities. Thus it is supposed that the vapor of peroxide of nitrogen is a gas-mixture, the components of which are represented (in the newer chemical notation) by NO2 and N2O4 respectively. The densities corresponding to these formulæ are 1.589 and 3.178. The density of the mixture should have a value intermediate between these numbers, which is substantially the case with the actual vapor. The case is similar with respect to the vapor of formic acid, which we may regard as a mixture of CH2O2 (density 1.589) and C2H4O4 (density 3.178), and the vapor of acetic acid, which we may regard as a mixture of C2H4O2 (density 2.073) and C4H8O4 (density 4.146). In the case of perchloride of phosphorus, we must suppose the vapor to consist of three parts; PCl5 (the proper perchloride, density 7.20), PCl3 (the protochloride, density 4.98), and Cl2 (chlorine, density 2.22). Since the chlorine and protochloride arise from the decomposition of the perchloride, there must be as many molecules of the type Cl2 as of the type PCl3. Now a gas-mixture containing an equal number