84 M. Achille Cazin on Internal Work in Gases. 



Sir W. Thomson has substituted for this principle the follow- 

 ing, which he has also taken as axiomatic : — It is impossible to 

 obtain work from the heat of a body without having another 

 body at a lower temperature to take from it a part of this 

 heat. These two principles do not constitute true axioms ; they 

 are, rather, experimental facts. M. Hirn has recently given a 

 new demonstration of the second fundamental principle, in which 

 he seeks to deduce it from the first principle without making 

 any hypothesis*. But even if this demonstration should rest, 

 like M. Rankine's, on some hypothesis, the truth of the second 

 fundamental principle cannot be doubted, since all its conse- 

 quences have hitherto been verified by experiment. 



The bases of the mechanical theory being perfectly verified, 

 it remains to be known whether the laws of Mariotte and Gay- 

 Lussac are sufficiently so. Now the celebrated experiments of 

 M. Regnault demonstrate their incorrectness. They can only 

 be accepted approximately for gases not liquefiable ; and they 

 must be completely rejected, even in technical applications, for 

 the other gases and superheated vapours, the use of which in 

 thermal motors cannot fail to spread now that the successes of 

 M. Hirn in this direction have been verified f. 



Hence the experiments of M. Regnault, and all those which 

 have suggested the two fundamental principles of the mechanical 

 theory, lead to the assumption of the existence of internal work 

 in gases. It remains to us to examine the direct demonstrations 

 of this property of gases. 



Everybody at the present day knows Mr. Joule's cele- 

 brated experiment in which a reservoir containing compressed 

 gas is put in communication with an empty reservoir J. It 

 is evident that the gas undergoes a change of volume and of 

 pressure without there being any external work effected. Con- 

 sequently a negative internal work will be accompanied by the 

 disappearance of a certain quantity of heat; and if, the reser- 

 voirs being surrounded by water, we observe a decrease in tem- 

 perature, we may from this conclude that the operation gives 

 rise to a like amount of work. Theoretically this experimental 

 process is not open to objection; yet it is not sufficiently delicate, 

 and it does not give definite results. From this it has natu- 

 rally been concluded that the internal work of gases may be 

 neglected. 



M. Hirn has tried to increase the delicacy of the apparatus by 



* G. A. Hirn, " Memoire sur la Thermodynamique," Annates de Chimie 

 et de Physique, S. 4. vol. x. p. 32 (May 186*7). 



f " Recherches experimentales sur les machines a vapeur," par M. G. 

 Leloutre, Bulletin de la Societe industrielle de Mulhouse, 1866. 



+ Phil. Mag. May 1845. 



