
HEAT PRODUCED BY THE COMPRESSION OF A GAS. 293 
in the expression for it being obtained by dividing the numbers tabulated at the end 
of Reanavut’s eighth Mémoire by 760. The considerableness of the deviations 
from the gaseous laws which equation (II.) indicates, is seen at once by comparing 
the numbers in Col. 2 with those in Col. 3 of the preceding Table, and observing 
that the coefficient of [¢] in (II.) is, for each temperature shown in that Table, 
obtained by dividing the corresponding number in Col. 2 by that in Col. 3. Col. 4 
shows what the values of would be if the density of saturated steam at 100° were 
re ul : ° 
imi7~ instead of jg53. and, for other temperatures, varied according to the gaseous 
laws. 
8. This subject has been very carefully examined by Cuaustus, who has indi- 
cated the great deviations from the gaseous laws of density that Mayer’s hypothesis 
requires in saturated steam, and has given an empirical formula for the density 
of saturated steam founded on that hypothesis, and on ReGNavut’s observations 
on the pressure and latent heat. In this direction theory can go no farther, for 
want of experimental data, although, from what we know of gases and saturated 
vapours, it may be doubted whether such excessive deviations, in the case of 
steam, from the laws of a “ perfect gas” are rendered probable by a hypothesis 
resting on no experimental evidence whatever.* 
9. To JouLE we are indebted for a most important series of experimental 
researches on the relation between the thermal effects, the external mechanical 
effects, and the internal mechanical effects (es viva destroyed by fluid friction) 
due to compressions and expansions of air in various circumstances. These re- 
searches afford actual tests which, so far as they go, are verifications of the 
truth of MAYeEr’s hypothesis for temperatures between 50° and 60° Fahr., founded 
on two distinct methods, either of which is perfect in principle, and might be 
made the foundation of experiments at any temperature whatever. 
10. The first of these methods consists simply in determining, by direct experi- 
ment, the heat evolved by the expenditure of a given amount of work in com- - 
pressing air, and comparing it with the quantity of heat created by the same 
amount of work in Jou.e’s original experiments on the heat developed by mag- 
neto-electricity, and by the friction of fiuids in motion. 
11. The second method is especially remarkable, as affording, in each experi- 
ment, an independent test of the truth of Mayer’s hypothesis for air at the tem- 
perature used, without requiring any knowledge of the absolute value of the me- 
chanical equivalent of heat. In Jouiz’s actual experiments, the test is simply 
* Joutx’s experimental verification of Mayer’s law for temperatures of from 50° to 60° Fahr., 
shews, if rigorously exact, that the density of saturated steam at about 10° centigrade must be te 
of what was assumed for it in the calculations of my former paper, but does not go towards indi- 
eating any deviation from the gaseous laws of variation in the density of saturated steam at different 
temperatures. 
¢ Philosophical Magazine, May 1845. 
VOL. XX. PART II. 4k 

