J. W. Gibbs — Vapor- Densities. 287. 
whole heat absorbed by the vapor we must add that which 
would be required if no conversion took place. At 40° the 
vapor of peroxide of nitrogen contains about 54 molecules of 
N,0, to 46 of NO,, as may easily be calculated from its density. 
The specific heat for constant pressure of a mixture in such 
proportions of gases of such molecular formule, if no chemi- 
eal action could take place, would be about twice that of the 
same volume of air. Adding this to the heat absorbed by the 
chemical action we obtain the final result,—that at 40° and the 
pressure of the atmosphere the specific heat of peroxide of 
nitrogen at constant pressure is about eighteen times that of 
the same volume of air.* 
increased liability to error in cases of this kind. e expan- 
sion of peroxide of nitrogen for increase of temperature under 
constant pressure at 40° is 3°42 times that of air. If then, ina 
determination of density, the vapor fails to reach the tempera- 
ture of the bath, the error due to the difference of the tempera- 
ture of the vapor and the bath, will be 3°42 times as great as 
would be caused by the same difference of temperatures in the 
e of any vapor or gas having a constant density. When we 
consider that we are liable not only to the same, but to a much 
greater difference of temperatures in a case like that of perox- 
ide of nitrogen, when the exposure to the heat is of the same 
uration, it is evident that the common test of the exactness of 
a process for the determination of vapor-densities, by applying 
it to a case in which the density is nearly constant, is entirely 
Insufficient. 
_ That the experiments of the III* series of Deville and Troost 
give numbers so regular and so much lower than the other 
experiments is probably to be attributed in part to the length of 
time of exposure to the heat of the experiment, which was half 
an hour in this series,—for the other series, the time is not given. 
nother point should be considered in this connection. 
During the heating of the vapor in the bath, it is not immate- 
rial whether the flask is open or closed. This will appear, if 
dD : , 
we compare the values of (>) and (2) , the differential 
p ° 
coefficients of the density with respect to the temperature on 
the suppositions, respectively, of constant pressure, and of con- 
stant volume. For 40°, we have 
dD 7) 168, -: 
(=) 70189, ( dt ee ? 
