AND THE CONSTITUTION OF ELASTIC FLUIDS. 109 



atmospheric pressure, into the space of 136^ cubic inches, 

 13°.63 of heat per lb. of water were produced; whereas 

 by the reverse process, of allowing the compressed air to 

 expand from a stop-cock into the atmosphere, only 4°.09 

 were absorbed instead of 13".63, which is the quantity of 

 heat which ought to have been absorbed, according to the 

 generally received hypothesis. I found, also, that when 

 strongly compressed air was allowed to escape into a vacuum, 

 no cooling effect took place on the whole, a fact likewise at 

 variance with the received hypothesis. On the contrary, the 

 theory I ventured to advocate * was in perfect agreement 

 with the phenomena ; for the heat evolved by compressing 

 the air was found to be the equivalent of the mechanical 

 power employed, and, vice versa, the heat absorbed in rare- 

 faction was found to be the equivalent of the mechanical 

 power developed, estimated by the weight of the column of 

 atmospheric air displaced. In the case of compressed air 

 expanding into a vacuum, since no mechanical power was 

 produced, no absorption of heat was expected or found. 

 M. Seguin has confirmed the above results in the case of 

 Ifkara. 



The above principles lead, indeed, to a more intimate 

 acquaintance with the true theory of the steam-engine; for 

 they have enabled us to estimate the calorific effect of the 

 friction of the steam in passing through the various valves 

 and pipes, as well as that of the piston in rubbing against 

 the sides of the cylinder; and they have also informed us 

 that the steam, while expanditig in the cylinder, loses heat 

 in quantity exactly proportional to the mechanical force 

 developed, t 



• I subsequently found that M. Mayer had previously advocated a 

 similar hypothesis, without, however, attempting an experimental demon- 

 stration of its accuracy Anruden of Woehler and Liebig for 1842 — ^May, 



1851. J. P. J. 



•)■ A complete theory of the motive power of heat has been recently 



