238 tf. W. Gibbs — Equilihrintn of Heterogeneous Substances. 



NOg and the other the formula N^O^ is the same as that derived 

 from the depth of the color on the supposition that the absorption of 

 light is due to one of the components alone, and is proportioned to 

 the separate density of that component.* 



MM. Sainte-Claire Deville and Troostf have given a series of deter- 

 minations of what we shall call the relative densities of peroxide of 

 nitrogen at various temperatures under atmospheric pressure. We 

 use the terra relative density to denote Avhat it is usual in treatises on 

 chemistry to denote by the term density, viz., the actual density of a 

 gas divided by the density of a standard perfect gas at the same 

 pressure and temperature, the standard gas being air, or more strictly, 

 an ideal gas which has the same density as air at the zero of the 

 centigrade scale and the pressure of one atmos])here. In order to 

 test our equations by these determinations, it will be convenient to 

 transform equation (320), so as to give directly the relation between 

 the relative density, the pressure, and the temperature. 



As the density of the standard gas at any given temperature and 



P 



pressure may by (263) be expressed by the formula -^—, the relative 



density of a binary gas-mixture may be expressed by 

 Now by (263) 



a t 

 7>= (m, +^2)-^. (326) 



^ ^ pv 



a^ 7n^ -\- ao ni^ =: — . (327) 



By giving to ^3 and m^ successively the value zero in these equa- 

 tions, we obtain 



O I ^2 



where D-^ and Z>2 denote the values of D when the gas consists 

 wholly of one or of the other component. If we assume that 



JJ,=2IJ„ (329) 



we shall have 



From (326) we have 



«i = 2a2. (330) 



m , -{- n/^ :=. JJ , 



*Salet, "Sur la coloration du peroxyde d'azote," Comptes Eendiis, vol. Ixvii, p. 488. 

 f Comptes Rendus, vol. Ixiv, p. 237. 



