206 Professor Joseph Larmor [May 1, 



to enroll among the few supreme classics of scientific knowledge, 

 Sadi Carnot's small tract of 1824, ' Reflexions sur la Puissance 

 Motrice du Feu ' ; lie found in 1845 that it was quite forgotten, 

 though they knew in the book-shops of the social and political writings 

 of his brother Hippolyte Carnot, ultimately his editor and biographer 

 (1878) in later years. 



If one had to specify a single department of activity to justify 

 Lord Kelvin's fame, it would probably be his work in connection 

 with the establishment of the science of Energy, in the widest sense 

 in which it is the most far-reaching construction of the last century 

 in physical science. Tliis doctrine has not only furnished a standard 

 of industrial values which has enabled mechanical power in all its 

 ramifications, however recondite its sources may be, to be measured 

 with scientific precision as a commercial asset ; it has also, in its 

 other aspect of the continual dissipation of available energy, created 

 the doctrine of inorganic evolution and changed our conceptions of 

 tlie material universe. A sketch of tlie early history of this doctrine 

 will illustrate tlie innate power and independence of Lord Kelvin's 

 thought, as well as in some degree his relations to his great pre- 

 decessors and contemporaries. 



The initial difficulty of the subject lay in the feature, entirely 

 novel to physical science, that in the inorganic world what we call 

 dissipation or scattering of energy is loss only in a subjective sense ; 

 it concerns only the energy " availa])le to man, for the production of 

 mechanical efi'ect," to use Thomson's own phrase of 1852.* We can 

 produce organised mechanical effect from diffuse energy such as heat, 

 which consists in tlie unregulated motion of a crowd of jostling 

 molecules, only by judicious guiding of its innate effort towards an 

 equilibrium, just as we can get power from a turbulent waterfall by 

 guiding the stream against a mill-wheel or turbine. But when the 

 average of the molecular motions has come to a steady equilibrium 

 throughout all parts of the material system, of which uniformity of 

 temperature is the criterion, all chance of arranging or guiding part 

 of its molecular energy into co-ordinated power available for our 

 operations on finite bodies has passed away. This is, roughly, the 

 rationale of the principle of Carnot. Yet the energy has not dis- 

 appeared ; it is still there, but it is uniformly diffused and so not 

 recoverable into the organised form of mechanical power. This 

 absolute conservation of the total energy is the principle of Joule, 

 which is the main experimental support of the presumption that all 

 energy is ultimately of the dynamical type. In a complete view of 

 physical transformations the two principles, of Carnot and of Joule, 

 have both to find their places. Here a fundamental perplexity con- 

 fronted and detained Lord Kelvin for some three years, 1847-50. 



* Math, and Phys. Papers, i. p. 505. 



