or STEAM-ENOIKES WITH DET SATUEATED STEAM. 179 
the temperature corresponding to total absence of elastic pressure. On that supposition 
the specific heat of air under constant pressure was predicted in 1850 as being probably 
0-24 of that of water, or thereabouts*; and M. Kegnault, in 1853, ascertained it by 
experiment to be 0-238. The long series of experiments by Messrs. Joule and Thom- 
son on the thermic effects of currents of elastic fluids f, have proved still more con- 
clusively that if the scale of absolute temperature and that of the perfect-gas thermo- 
meter differ, it must be by quantities so small that they have not yet been measured. 
The expansion of a perfect gas from 32° Fahe. to 212° Fahr. being in the ratio 
1:1-365, the absolute zero is 
;^ = 493°-2 Fahr. 
O*oo5 
below the temperature of melting ice ; or 
t in degrees of Fahr. = 461°-2-1-T, 
T being the temperature on the ordinary Fahrenheit’s scale. 
is a function which remains constant when the mass under consideration eithei pei- 
forms work by expansion, or undergoes compression, without receiving or emitting heat. 
Its value is 
A:.hyp. logi^+J^^Zij; (2-) 
where k is the real specific heat of the substance, expressed in foot-pounds of energy 
per degree of temperature ; p is the elastic pressure of the mass per unit of area when 
it occupies the volume v ; the differentiation ^ is performed on the supposition of v 
being constant, and the integration on the supposition of t being constant. 
Another form of the function <p, which is convenient in certain calculations, is as 
follows : — 
log 
where — is the constant value of the ratio ^ for the substance under consideration, in 
k ^ 
the perfectly gaseous state J. 
The function <p is sometimes called the Thermo-dynamic function. 
One of its properties is as follows : — that when the series of changes of pressure and 
volume to which the integration of equation 1 is applied constitute a cycle., so that the 
mass returns in the end to its primitive volume and pressure, then for a complete cycle 
^f-t;)d(p=^{(p,—<p^)dt=l{p,-p^)dv-l{v,—v^)dp, .... (4.) 
* Edinburgli Transactions, vol. xx. f Philosophical Transactions, passim. 
+ /c+Wh is the specific heat of the substance under constant pressure, in the perfectly gaseous state, 
^0 
expressed in units of energy. 
2 B 2 
