which we call Heat. 111 
which we call the absolute temperature. Hence, according to 
the above, it follows that the absolute temperature is propor- 
tional to the vis viva of the translatory motion of the molecules. 
But as, according to a former remark, the several motions in 
one and the same gas bear a constant relation to each other, it is 
evident that the vis viva of the translatory motion forms an ali- 
quot part of the total vis viva, so that the absolute temperature 
is also proportional to the whole vis viva in the gas. 
These considerations, together with others connected there- 
with to be given hereafter, induced me, in my memoir “ On the 
Moving Force of Heat,” to express the opinion that the specific 
heat of gases was constant; which opinion was in opposition to 
the experiments then known*. The quantity of heat which 
must be imparted to a gas, under constant volume, in order to 
raise its temperature is to be considered as the increase of the 
vis viva in the gas, inasmuch as in this case no work is done 
whereby heat could be consumed. The specific heat wnder con- 
stant volume, therefore, is in a perfect gas the magnitude which 
Rankine calls the true specific heat. Now the assertion that the 
true specific heat of a gas is constant, is simply equivalent to the 
assertion that the total vis viva in the gas has a constant ratio to 
the vis viva of the translatory motion which serves us as a mea- 
sure of the temperature. With respect to the specific heat under 
constant pressure, I have proved in the memoir before cited, and 
by means of a hypothesis proceeding from the same considera- 
tions, that it differs only by a constant magnitude from the true 
specific heat. 
5. The foregoing is true for permanent gases only, and even 
for these only approximatively. In general, the small deviations 
which present themselves can be easily accounted for. 
In order that Mariotte’s and Gay-Lussac’s laws, as well as 
others in connexion with the same, may be strictly fulfilled, the 
gas must satisfy the following conditions with respect to its mo- 
lecular condition :— 
(1) The space actually filled by the molecules of the gas must 
be infinitesimal in comparison to the whole space occupied by 
the gas itself. 
(2) The duration of an impact, that is to say, the time required 
to produce the actually occurring change in the motion of a mo- 
lecule when it strikes another molecule or a fixed surface, must 
be infinitesimal in comparison to the interval of time between 
two successive collisions. 
(3) The influence of the molecular forces must be infinitesimal. 
Two conditions are herein involved. In the first place, it is 
requisite that the force with which all the molecules at their 
* Poggendorff’s Annalen, vol, lxxix. p.393, Phil, Mag. vol. ii. pp. 1, 102, 
