1908] on the Scientific Work of Lord Kelvin. 215 



forme. C'est im mouvement dans les particules des corps. Parfcout 

 ou il y a destruction de puissance motrice, il y a en meme temps, pro- 

 duction de chaleur," and reciprocally. Like Thomson at the later 

 date, he intended to seek the guidance of further experiment, outlines 

 of which he sketched. These extracts suggest the very problems 

 which are still fundamental in the molecular theory of Energetics, 

 about which much is yet to be learned, though Thomson's theory of 

 dissipation of energy and its molecular interpretation by Maxwell and 

 Thomson and Boltzmann has illuminated the whole field. Yet Carnot 

 already saw (p. 93) that his negation of perpetual motion demands 

 that when heat does work in falling to a lower temperature, if some 

 heat is really absorbed in the process the amount so absorbed must 

 be independent of the mechanism of the process, must, in fact, be an 

 equivalent of the work ; for if the other alternative were possible, 

 " on pourrait creer de la puissance motrice sans consommation de 

 combustible et par simple destruction de la chaleur des corps." 

 Clausius and Thomson had nothing in 1850 to add to this reasoning 

 of date earlier than 1832. 



No apology is required for thus dwelling at length on this episode 

 in the evolution of the principles of physical science, the development 

 of the principle of energy into its wider aspect, in which it assumes 

 its universal co-ordinating role as the principle of available energy — 

 involving its complete available conversation only in the limited class 

 of phenomena that satisfy the Carnot test of being reversible, and in 

 other cases emphasising the partial dissipation into diffused unavail- 

 able molecular energy which is characteristic of the operations of 

 physical nature. No passage in the history of modern physics can, 

 perhaps, compare with it in interest. In the other outstanding advance 

 of the last century, the unravelment of the function of the aether as 

 the sole means of intercommunication between the molecules of matter 

 so as to constitute a cosmos, as the seat of the activities of radiation 

 and of electric and chemical change, the problem to be solved was of 

 a different type. The questions have there been more precise ; they 

 have suggested, and their investigation has been directed by, definite 

 adaptable trains of experiment. But the pioneers in the theory of 

 available energy had to probe among the circana of common experi- 

 ence, in a manner which takes us back to the beginnings of dynamical 

 science and recalls the efforts of Archimedes and Galileo and Pascal 

 in detecting controlling principles in the maze of everyday phenomena. 



The original stimulus to all this wide grasp of the relations of 

 inanimate nature had its origin in the progress of mechanical inven- 

 tion, in the successful construction and operation of thermal engines. 

 Irrespective of the problem of their industrial improvement, the de- 

 tection of the essential features of this mechanical value of heat would 

 appeal strongly to an analytical mind like that of Carnot. But his 

 compact informing principle, as its content was ultimately developed in 

 Thomson's hands, far transcended the special thermal problem from 



