Prof. Tyndall on Mayer, and the Mechanical Theory of Heat \ 175 



(for I here trust my memory merely) that the assumption has 

 been completely verified by the experiments of the very philo- 

 sophers who once questioned it. "The law/' says Mayer, 

 " ( Heat = Mechanical effect/ is independent of the nature of an 

 elastic fluid, which only serves as the apparatus by means of 

 which the one force is converted into the other." 



The law of Mariotte was an old principle when Mayer wrote; 

 and the fact of its holding good for gases generally renders the 

 conclusion exceedingly probable that, in yielding to compression, 

 the attractions of the gaseous molecules were insensible ; other- 

 wise it is hardly conceivable that the same results could have 

 been obtained with gases so differently constituted : the attrac- 

 tions of the hydrogen atoms, for example, would in all pro- 

 bability be different from those of oxygen. Mayer was further 

 justified in his hypothesis, as to the absence of interior work in 

 the case of a true gas, by the experiments of CErsted and Des- 

 pretz, which showed that the law of Mariotte was departed from 

 by the liquefiable gases— the amount of departure depending on 

 the proximity of the gas to its point of condensation. Where, 

 therefore, no departure from the law had been observed (in the 

 case of air for instance), Mayer, I submit, was perfectly war- 

 ranted in assuming that the molecular attractions were insen- 

 sible, and that the quantity of heat (y) before referred to was 

 entirely expended in raising the weight, and had its true mecha- 

 nical equivalent in the weight so raised. 



With reference to the application of the mechanical theory 

 of heat to cosmical phenomena, if it were not a liberty, I would 

 ask whether you have ever read the essay of Mayer entitled 

 Beitrage zur Dynamik des Himmels ? If so, then I have good 

 reason to suspect my competence to come to a correct conclusion 

 as to what constitutes a scientific right. 



Knowing that the original memoirs of Mayer would be the 

 true court of appeal in connexion with this subject, I some 

 months ago urged the responsible editor of the Philosophical 

 Magazine to publish translations of them. This I hope he will 

 do ; for I quite agree with you in thinking that they would in- 

 terest many of the readers of the Magazine. Let me add, in 

 conclusion, that I do not think the public estimate of your 

 labours can be in the least affected by any recognition which 

 may be accorded to Mayer. There is room for both of you on 

 this grand platform. Certainly, had Mayer never written a 

 syllable on the mechanical theory of heat, I should not deem 

 your work a whit nobler than I now hold it to be. 



Believe me, 



Royal Institution, Yours, &c, 



August 1862. John Tyndall. 



