146 .Mr. J. P. Joule on the Dynamical Theory of Heat. 



words in reference to M. Mayer's letter to Dr. Tyndall, inserted 

 in your last Number, in which he says that he regarded the 

 question of priority as to the mechanical equivalent of heat as 

 exhausted by his communication to the Academy of Sciences in 

 Paris, inasmuch as I did not reply to him. That I did not so 

 reply to M. Mayer was not because I was unable to do so, and 

 is still less to be attributed to any disrespect for the celebrated 

 German physicist, but was simply because his communication, 

 being a statementof opinion contrary to mine,did not appear to call 

 for a rejoinder, which would probably have led to an unprofitable 

 controversy. I have found no reason to alter my own view, as 

 stated in my article in the Comptes Rendus for 1849, part 1. p. 132, 

 to which M. Mayer's communication was a reply, which view is 

 stated as follows at p. 134 : — "Avant mes experiences, il n'y 

 avait aucuns faits sur lesquels on put avec certitude baser la 

 conclusion que la chaleur specifique d'un gaz est la meme dans 

 ses divers etats de densite. Au contraire, l'opinion generale, 

 conformement aux experiences de MM. De la Rive et Marcet, 

 etait que la chaleur specifique d'un gaz varie avec la pression k 

 laquelle il est soumis; d'ou. il decoule que la conclusion non 

 appuyee de M. Mayer, qui n'est pas en concordance avec les 

 faits connus a cette epoque, n'avait pas du appeler l'attention 

 des savants." 



In M. Mayer's reply he remarks — "M, Gay-Lussac a de- 

 montre qu'un gaz coulant d'un ballon dans un autre vide et 

 d'une capacite egale, se refroidit dans le premier autant qu'il se 

 chauffe dans le second. II s'ensuit evidemment de cette experi- 

 ence que la chaleur specifique d'un gaz n'est pas alteree par la 

 rarefaction/' 



I have not been able to consult the first volume of the Memoires 

 d'Arcueil, which contains the description of Gay-Lussac's experi- 

 ment; but it is certain that the result of it, as stated by M. 

 Mayer, was not received or remembered by the scientific world, 

 so that M. Regnault and myself were considered by others, as 

 we considered ourselves, to be original inquirers on this subject. 

 Moreover, it may be gathered from M. Mayer's papers that he 

 knew nothing of this experiment of Gay-Lussac when he wrote 

 his celebrated memoir of 1842. This, however, so far from 

 detracting from the merit of M. Mayer, must, on the contrary, 

 increase our admiration of his wonderful prescience. But at the 

 same time this merit (and Dr. Tyndall agrees with me in this 

 point) cannot in the smallest degree interfere with the merit of 

 those who either before M. Mayer, or after him, pursued the 

 course of strict logical deduction from facts ascertained by others 

 and by themselves — the method by which alone the fabric of 

 science is securely built. 



