OF GASES AND VAPOURS. 195 
If this relation is independent of the temperature, there results 
simply eo constant and 
F, = A + b t 
in which A and @ are constants. 
Moreover, the direct experiments of Gay-Lussac and Welter 
are in favour of this assumption, who found the relation under 
consideration independent of the temperature or pressure; and 
also the coincidence of the values of the specific heats ascer- 
tained by De la Roche and Bérard with those of other observers, 
notwithstanding that they submitted the gases in the experiment 
to 100° C., while in the observations of others the gases had 
only a temperature of 10° to 20°. 
The experiments of Dulong and Petit on the refrigeration by 
gases, likewise show that the specific heat of gases is independent 
of the temperature ; these philosophers found that the cooling 
varies, indeed, with the pressure, but not with the temperature 
of the gases. 
From all this it results that we are justified, in the present 
state of science, in substituting A + b¢ for the function F,, at 
least as a very close approximation, 
7. The formulz of the preceding paragraphs now become 
yeaa pete dy 5 By] 
a Po 
aegy aide 1G Be adh as “mica bah 
a Po 
a Po a 
8. We have still to determine the arbitrary quantity a. This 
being arbitrary, the unit of heat (2.) dependent on it may itself 
be selected at wjll, and a, conversely, be determined by it. I 
therefore take, as is usually done, the quantity of heat which 
is capable of heating 1 kil. water at 0° to 1° as unit of heat. 
According to the determination of De la Roche and Bérard, 
the specific heat of the atmospheric air, with a pressure of one 
atmosphere, is equal to 0-267 of the unit of heat just established. 
If we take p, = to the pressure of an atmosphere, we obtain 
NL. . = 0, and therefore for atmospheric air c = 6 = 0:267. 
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