V. 



ON THE VAPOR-DENSITIES OF PEROXIDE OF NITROGEN, 

 FORMIC ACID, ACETIC ACID, AND PERCHLORIDE OF 

 PHOSPHORUS. 



[American Journal of Science, ser. 3, vol. xvm, Oct.-Nov. 1879.] 



THE 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 N0 2 

 and N 2 O 4 respectively. The densities corresponding to these formulae 

 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 

 CH 2 O 2 (density T589) and C 2 H 4 O 4 (density 3178), and the vapor 

 of acetic acid, which we may regard as a mixture of C 2 H 4 O 2 

 (density 2'073) and C 4 H 8 O 4 (density 4146). In the case of per- 

 chloride of phosphorus, we must suppose the vapor to consist of 

 three parts; PC1 6 (the proper perchloride, density 7'20), PC1 3 (the 

 protochloride, density 4'98), and C1 2 (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 C1 2 as of 

 the type PC1 8 . Now a gas-mixture containing an equal number 



* The language of this paper will be conformed to this usage. 



