382 VAPOR-DENSITIES. 



heat of peroxide of nitrogen at constant pressure is about eighteen 

 times that of the same volume of air.* 



But the greater amount of heat which is required to bring the 

 vapor to the desired temperature is only one factor in the increased 

 liability to error in cases of this kind. The expansion of peroxide 

 of nitrogen for increase of temperature under constant pressure at 

 40 is 3'42 times that of air. If, then, in a determination of density, 

 the vapor fails to reach the temperature of the bath, the error due 

 to the difference of the temperature 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 case 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 peroxide of nitrogen, when the exposure to the heat is of the same 

 duration, 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 d 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. 



Another point should be considered in this connection. During 

 the heating of the vapor in the bath, it is not immaterial whether 

 the flask is open or closed. This will appear, if we compare the 



values of f -^-J and (-53 ) the differential coefficients of the density 



with respect to the temperature on the suppositions, respectively, of 

 constant pressure, and of constant volume. For 40, we have 



= 0163, 



i 



the first number being obtained immediately from equation (10) by 

 differentiation, and the second by differentiation after substitution 



/' in f 

 of |x- for p. The ratio of these numbers evidently gives the 



proportion in which the chemical change takes place under the two 

 suppositions. This shows that only about six-sevenths of the heat 

 required for the chemical change can be supplied before opening 

 the flask, and the remainder of this heat as well as that required 

 for expansion must be supplied after the opening. The errors due 



* Similar calculations from less precise data for the bromhydrate of amylene at 225 

 seem to indicate a specific heat as much as forty times as great as that of the same 

 volume of air. 



