384 Prof. TyndalPs Remarks on the 



conclusions from slender premises ; while the Englishman aimed, 

 above all things, at the firm establishment of facts. And he did 

 establish them. The future historian of science will not, I think, 

 place these men in antagonism. To each belongs a reputation 

 which will not quickly fade, for the share he has had, not only 

 in establishing the dynamical theory of heat, but also in leading 

 the way towards a right appreciation of the general energies of 

 the universe." 



It will be observed that Mayer implies in his calculation that 

 the excess of heat imparted to the gas when it expands under 

 constant pressure is all consumed in the lifting of the weight, 

 that there is none of it expended in overcoming the mutual 

 attractions of the gaseous molecules. Whether Mayer, at the 

 time he made his calculation, had really fixed his attention on 

 the possibility of some of the heat being consumed in surmounting 

 these attractions does not appear from his first paper; all that 

 appears there is, that he fixed upon a substance (atmospheric 

 air) which was capable of giving a correct result. There were, 

 moreover, existing at the time data which made it all but certain 

 that atmospheric air was a body of this character. It obeyed 

 the law of Mariotte, its density being through a large range pro- 

 portional to the external pressure. If the condensation of the air 

 had been in part due to the mutual action of its own atoms — 

 if the external pressure and internal attractions had been thus com- 

 plicated — it is all but certain that the law of Mariotte could not 

 have been obeyed. Besides, the same law applied to hydrogen as 

 to air ; hence if the atoms exercised any sensible influence upon 

 each other, the hydrogen atoms must be assumed to act precisely 

 in the same manner as the oxygen atoms. It would also have to 

 be assumed that the action of two molecules of the same gas 

 exert precisely the s^jne action upon each other as two molecules 

 of different gases*. Gay-Lussac's experiments had, moreover, 

 proved that in the expansion of air, where no external work was 

 performed, no heat was absorbed, which could not be the case if 

 molecular attractions had to be overcome. In short, confining 

 ourselves to the data known in 1842, the hypothesis that any 

 sensible portion of the heat communicated to the air is applied 

 to the overcoming of internal attractions is cumbered with diffi- 

 culties and improbabilities sufficient to justify its instant rejec- 

 tion, How far Mayer entered into these considerations in 1842 

 we know not; all that we know is, that what he did was correct. 

 It is not a lt reconstruction or destruction of thermo-dynamics" 

 that those considerations involve, but simply a reconstruction of 

 Professor William Thomson's ideas regarding Dr. Mayer f. 



* On this point see also Verdet, p. 44. 



t If my knowledge be correct, the circumstances which might affect the 



