THERMAL EFFECTS OF FLUIDS IN MOTION. 
343 
of atmospheric air, according- to the result obtained above, is — , and in the case of 
carbonic acid — • 
32 
It is important to observe how much less close is the compensation in carbonic 
acid than in either of the other gases, and it appears probable that the more a gas 
deviates from the gaseous laws, or the more it approaches the condition of a vapour 
at saturation, the wider will be the discrepancy. We hope, with a view to investi- 
gating further the physical properties of gases, to extend our method of experiment- 
ing to steam (which will probably present a large cooling effect), and perhaps to 
some other vapours. 
In Mr. Joule’s original experiment* to test the relation between heat evolved and 
work spent in the compression of air, without an independent determination of the 
mechanical equivalent of the thermal unit, air was allowed to expand through the 
aperture of an open stopcock from one copper vessel into another previously exhausted 
by an air-pump, and the whole external thermal effect on the metal of the vessels, and 
a mass of water below which they are kept, was examined. We may now estimate the 
actual amount of that external thermal effect, which observation only showed to be 
insensibly small. In the first place it is to be remarked, that, however the equi- 
librium of pressure and temperature is established between the two air vessels, pro- 
vided only no appreciable amount of work is emitted in sound, the same quantity of 
heat must be absorbed from the copper and water to reduce them to their primitive 
temperature ; and that this quantity, as was shown above, is equal to 
PY P-P' 27000 x -0024 P-P' P-P' 
-J X -0024 x — Q-= 7T7T7, X uf —'046^ 
1390 
n 
n 
In the actual experiments the exhausted vessel was equal in capacity to the charged 
vessel, and the latter contained ’13 of a pound of air under 21 atmospheres of pressure, 
at the commencement. Hence P'=x P, and 
P-P' 
n : 
10-5 ; 
and the quantity of heat required from without to compensate the total internal 
cooling effect must have been 
•046 X 10-5X'13 = ‘063. 
This amount of heat, taken from 16^ lbs. of water, 28 lbs. of copper, and 7 lbs. of 
tinned iron, as in the actual experiment, would produce a lowering of temperature 
of only ‘003° Cent. We need not therefore wonder that no sensible external thermal 
effect was the result of the experiment when the two copper vessels and the pipe 
connecting them were kept under water, stirred about through the whole space 
* The second experiment mentioned in the abstract published in the Proceedings of the Royal Society, June 
20, 1844, and described in the Philosophical Magazine, May 1845, p. 377. 
