SW. Gibbs — Vapor-Densities. 281 
PF.” v 
rn. == n= == 
1 > oo" > 
at sa,t 
where a, denotes a constant, we have 
log i= — (A + log 2 a,) — (1+ B) logt + < 
= - A’— Blog +<, (5) 
where A’ and B’ are new constants. Now if we denote by p 
the total pressure of the gas-mixture (in millimeters of mer- 
cury), by D its density (relative to air of the same temperature 
and pressure), and by D, the theoretical density of the rarer 
component, we shall have 
p:p+p,::D,:D. 
This appears from the consideration that p+p2 Di hey what 
the pressure would become, if without change of temperature 
or volume all the matter in the gas-mixture could take the form 
of the rarer component. Hence, 
2 
Pi Pee D2) 
“D, (D—D,) 
and P, = 1 ae : 
yp; p(2D,—D) 
By substitution in (5) we obtain 
Di(D—D) __ 4r_Brtoge + S + logy. (6) 
CE AMEON eae peri tes MRE Pao 
By this formula, when the values of the constants are deter- 
mined, we may calculate the density of the gas-mixture from 
its temperature and pressure. The value of D, may be obtained 
from the molecular formula of the rarer component, If we 
compare equations (8), (4) and (5), we see that 
C; — Cy 
B=B+1, B= 56 ‘ 
log 
Now ¢,—cy is the difference of the specific heats at constant vol- 
ume of NO, and NO, The general rule that ming ah ee 
of a gas at constant volume and per unit of weight is Pret e- 
endent of its condensation, would make ¢,=¢, B=0, er a 
t may easily be shown, with respect to any of the su 
stances considered in this paper,* that u ith - 
value of B’ greatly exceeds unity, the term B’ log ¢ may Ps 
neglected without serious error, if its omission 1s compensate 
* For the case of peroxide of nitrogen, see pp. 243, 244 in the paper cited above. 
