278 GREAT MEN OF SCIENCE 



in Mayer's edifice.^ Nevertheless, the system set up 

 by Mayer gave from the start the impression of being true 

 to nature; for it threw Hght upon so many processes in 

 nature that had hitherto been completely in darkness; it 

 provided such a wide-ranging summary of knowledge, as 

 would not have been possible if any of its main points were 

 not correct. Just as in the case of Newton's structure of 

 ideas in the Principia, where all the knowledge of nature col- 

 lected by his most eminent predecessors provided proofs 

 in abundance for his system, so also in the case of Mayer's 

 system of ideas, where the proof was provided by the 

 various investigations we have mentioned. 



However, Mayer himself had already undertaken a com- 

 parison with reality in certain special cases, and here he pro- 

 ceeded quantitatively. He calculated the number of metre 

 kilograms (potential energy) resulting from the transforma- 

 tion of one calorie of heat - in other words the mechanical 

 equivalent of heat - by using existing measurements of the 

 specific heat of air at constant pressure and constant volume 

 in an entirely new manner. The fact, grasped by Dalton and 

 confirmed by Gay-Lussac and Laplace, that air is cooled 

 when it expands against external pressure, is regarded by 

 Mayer as a case in which the heat which disappears has been 

 turned into work, which is done against external pressure, 

 and he based the admissibility of this view upon the fact 

 that a gas when expanding into a vacuum -that is to say 

 without doing work - does not show any cooling, as had 

 already been proved by Gay-Lussac in the year 1807. So 

 that, in fact, the performance of work and consumption of 

 heat are connected together in the closest possible way.^ 



1 We might mention as a necessary correction made afterwards the 

 introduction of the factor i into the kinetic energy. 



2 Among many statements brought forward against Mayer's view, and 

 one which was only with difficulty and after some time cleared up, smce 

 it was brought forward by persons of apparent authority, is the statement 

 that his calculation of the mechanical equivalent of heat is not free from 



