the Mechanical Theory of Heat. 471 



first meeting to explain to you the transformation of heat into 

 mechanical power*. 



I now come to the labours, from 1842 to 1849, which have de- 

 finitely established the science. These labours are the exclusive 

 work of three men, who without concert, without even knowing 

 each other, arrived at the same time, and almost in the same 

 manner, at the same ideas. The priority in the order of publi- 

 cation belongs without any doubt to the German physician, 

 Jules Robert Mayer, whose name has occurred so often in these 

 discourses ; and it is interesting to know that it was in reflecting 

 on certain observations in his medical practice that he conceived 

 the idea of the necessity of a relation of equivalence between work 

 and heat. The variations of the difference of colour of arterial 

 and venous blood directed his attention to the theory of respira- 

 tory phenomena. He soon saw in the respiration of animals the 

 origin of their motive power ; and the comparison of animals to 

 thermic machines afterwards suggested to him the important 

 principle with which his name will remain for ever connected. 

 Such is the account which he gives himself of the development 

 of his ideas, in his memoir "On Organic Motion and Nutrition," 

 published in 1845. His first paper, entitled "Remarks on the 

 Forces of Inanimate Nature," published in 1842 in Liebig's 

 Annalen, does not, however, contain any allusion to vital pheno- 

 mena, and deduces simply the equivalence of heat and work from 

 the comparative study of friction, of the steam-engine, and of the 

 properties of gases. 



We find moreover in this memoir a first determination of the 

 mechanical equivalent of heat deduced from the properties of 

 gases, perfectly exact in principle, but the result of which, in 

 consequence of the inaccurate values of the coefficient of expan- 

 sion and the specific heat of air which were current in science 

 twenty years ago, is considerably wide of the truth. The memoir 

 " On Organic Motion and Nutrition," and the •' Celestial Dyna- 

 mics," published in 1848, contain physiological and astronomical 

 applications of the new principle, and show that, notwithstanding 

 a scientific education imperfect on many points, Mayer compre- 

 hended the bearing of his discovery, and knew how to follow it up. 

 Towards the epoch of the first publication of Mayer, M. Cold- 

 ing, Hydraulic Engineer of the city of Copenhagen, presented 

 to the Royal Scientific Society of Denmark a series of memoirs 

 on the power of the steam-engine and gas-engine, which con- 

 tained ideas almost identical with those of Mayer, and an ex- 

 perimental determination of the mechanical equivalent of heat 

 by friction, which does not appear to be very exact. These 

 titles suffice to assure to M. Colding a place among the disco- 

 verers of the new theory. But we ought to remember that the 

 * Etude sur l' influence des chemins defer, p. 380. 



