34 
MR. J. \V. CAPSTICK OX THE RATIO OP THE SPECIFIC HEATS 
§ 15. Discussion of the Results. 
Gathering the results together, we have the following table :—- 
Table XXVI. 
Name. 
Formula. 
7- 
1 Methane. 
CH, 
1-313 : 
Methyl chloride. 
CH 3 CI 
1-279 
Methyl bromide. 
CH^Br 
1-274 
Methyl iodide. 
OH 3 I 
1-286 
Ethane. 
C 0 H 3 
1-182 
Ethyl chloride. .. 
aHjCi 
1-187 
Ethyl bromide. 
CoH^Br 
1-188 
Propane . .. 
C. 3 H« 
1-130 
Normal propyl chloride. 
O 3 H 7 CI 
1-126 
Isopropyl chloride. 
C 3 H 7 CI 
1-127 
Isopropyl bromide. 
CsH^Br 
1-131 
i 
It will be seen on referring back to the separate results for methyl and ethyl 
chlorides and a few others of the gases that the values of y are slightly higher at the 
higher pressures. This circumstance suggests a doubt as to the lawfulness of taking 
the mean, for if the change were at all considerable, the right thing to do would be to 
extend the range of the observations till a constant value was reached, and use this 
value for comparison. The change observed can hardly be said'to be beyond the 
range of experimental error in any case, and is perhaps only accidental. 
The only experiments I am acquainted with that have been made to test the 
question whether y varies with the pressure or not are those of Jager (‘ Wied.,’ vol. 36, 
p. 165), who concluded that it does not. His results for ether vapour show close 
concordance at saturation and half saturation, but the discordance of the results for 
alcohol and water lessens the value of those for ether. 
The specific heat at constant pressure includes the change of potential energy due 
to separation of the molecules, and hence y will probably not be quite independent of 
the pressure, if the gas does not obey Boyle’s Law,^'' but if the change in y is due 
only to this, it is not likely to be great. 
The question has arisen quite incidentally in my work, for I was not looking for 
any such effect, and did not plan the experiments so as to make it perceptible. I 
have in no case used a very long range of pressures, and have always avoided going 
near saturation, where the effect might be expected to be most noticeable. 
The point is one that, ought to be settled. Meanwhile, the obvious law to which 
* See Pi'ofesser Fjtzgrrald, ‘ Roy. Soc. Proc.,’ rol. 42, p. 50. 
