208 Professor Joseph Larmor [May 1, 



with it every other branch of the Theory of Heat, may ultimately 

 require to be reconstructed upon another foundation when our ex- 

 perimental data are more complete." He returns, in a note, stimulated 

 by a remark of Joule, to the problem of what becomes of the me- 

 chanical effect that appears to be lost when heat diffuses ; but he 

 cannot admit the suggestion of Joule to cut the knot by abandoning 

 Carnot's principle, and he appeals to further experiment " either for 

 a verification of Carnot's axiom, and an explanation of the difficulty 

 we have been considering : or for an entirely new basis for the 

 Theory of Heat." Still harassed by these doubts, he returns yet 

 again to test the experimental verification of Carnot's principle (which 

 he finds adequate) in an Appendix ; * for, as he says, " Nothing in the 

 whole range of Natural Philosophy is more remarkable than the 

 establishment of general laws by such a process of reasoning " as is 

 that principle in its wider ramifications. 



We have here found Thomson actually hesitating as to whether 

 heat is to be classified as energy, on the ground that the fall of heat 

 to a lower temperature can occur without developing any mechanical 

 work. Yet it is true, as Lord Rayleigh has expressed it, f that most 

 great authorities, especially in England, including Newton, Cavendish, 

 Rumford, Young, Davy, etc., have always been in favour of the 

 doctrine that heat is a mode of motion. The fact is, as we have 

 seen, that Thomson knew too much to allow him to rest in such a 

 partial view of things ; he saw also a totally different side of the 

 subject, which not even his close connection with Joule and appre- 

 ciation of his work, could allow him merely to ignore. 



Just a year before Thomson's first paper on Carnot's principle, 

 Helmholtz, then a young army surgeon, had stepped (1847) into the 

 first rank of physicists (though recognition came later, the memoir, 

 e.g., becoming known to Thomson only in 1852) by the publication 

 of the ' Erhaltung der Kraft,' which asserted the universality of the 

 conservation of total energy, and developed with convincing terseness 

 and lucidity the ramifications of that principle throughout nature. 

 To establish the transformation of heat into work he is already able 

 to appeal to the classical experiments of Joule, published three years 

 previously (1844) — not yet mentioned by Thomson, whether it was 

 from want of knowledge or from some fancied mode of evading their 

 force in the light of his insistence on Carnot's principle. These ex- 

 periments proved definitely that expansion of a gas working against 

 the pressure of the atmosphere absorbs an equivalent of heat, whereas 

 expansion into a vacuum absorbs »none. It was, in fact, in this paper 

 that Joule rather summarily condemned Carnot's principle as above 

 mentioned, on account of its supposed discrepancy with his own 

 established results. And Helmholtz had naturally to consider this 



* April 30, 1849. 



t The Scientific Work of Tyndall, Roy. Inst. Proc, Vol. xiv. p. 218. 



