J. W. Gibbs — Vapor-Densities. 277 
will not delay in being universally accepted by competent 
judges,” 
Art. XXXVII.— On the Vapor- Densities of Peroxide of Nitrogen, 
Formic Acid, Acetic Acid, and Perchloride of Phosphorus ; by 
J. WILLARD GIBBS. 
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 
nown 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 den- 
sity taken relatively to air of the same temperature and pres- 
sure,* 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 com- 
prised between zero and 200° or 800° of the centigrade scale. 
Such anomalies have been explained by the supposition that 
the vapor consists of a mixture of two or three di erent kinds 
of gas or vapor, which have different densities. Thus it is sup- 
posed that the vapor of peroxide of nitrogen is a gas-mixture, 
the components of which are represented (in the newer chemical 
notation) by NO, and N,O, respectively. The densities cor- 
responding to these formulz are 1589 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, 
eects may regard as a gets CH,O, epee 1589) 
and ©,H,O, (density 3-178), and the vapor of acetic acid, 
which we hh re a as a Mane of C,H,O, (density 2°073) 
and C,H,O, (density 4-146). In the case of perchloride of 
phosphorus, we must suppose the a to consist of three 
parts ; PCI, (the proper perchloride, density 7-20), PCls (the 
aces aed density 4-98), and Cl, (chlorine, density 2-22). 
ince the chlorine and protochloride arise from the decom- 
* The language of this paper will be conformed to this usage. 
