162 Prof. Barnard on the comparative Expenditure of Heat 
higher than to 240. The maximum temperature usually em- 
ployed was 650° F. 
practicable. In Ericsson’s, the compression cylinders of Joule, 
and the regenerators of Stirling, have been united; but I have 
shown that the regenerators are not essential to economy. It has 
however, become a matter of interest to compare with each other 
the several forms which it has been proposed to give to the aif 
2d. That when heat is absorbed by a solid or a liquid, it 1s # 
part expended in the internal mechanical effect of overcoming 
cohesion, and in part in the external effect of pressure upon sur 
rounding resistances; while in part it remains sensible: but that, 
in the case of a perfect gas, there is no internal expenditure; 8 
et atmos 
pheric air, and those aériform bodies generally, which are a 
reducible by pressure and cold to the liquid state, are sensibly an 
_ 5th. That equal volumes of different gases, however differed 
in density or specific heat, when taken at the same pressure an 
4th. That though no gas is perhaps rigidly perfect, y 
* Mr. Joule’s severe investigation of this point has fixed the “mechanical equ 
lent” of a “unit of heat”—that is, of the quantity of heat which would raisé 
p ift 
temperature of a of water one degree at 77 ifted one foot. that 
t The properties of a perfect gas are expressed in the law of Mariotte, ViZ of 
the pressure is i I e volume, tem e that 

eS >; an . 
ay ane, thatthe premare te directly asthe tamporaare above de 
volume iz 
