34 HERMANN VON HELMHOLTZ 



The five months of his stay in Berlin were spent in hard 

 work, and in close scientific intercourse with his friends du 

 Bois-Reymond and Briicke, who, when separated later on, 

 communicated all their scientific projects, and the results of 

 their work, to each other in writing. This was the first 

 Christmas that Helmholtz had not spent with his parents. 

 He was doing his clinical tests at the Infirmary, and at 

 the same time working steadily in the laboratory of Magnus 

 at his researches on fermentation and putrefaction, while he 

 regularly attended the meetings of the Physical Society. 



In January, 1846, he announces in a letter to his parents 

 that he had done well in the examination as physician and 

 surgeon, but did not pass as an ' operator '. 



Immediately after his return to his military duties at Potsdam 

 we find him busy again with his experiments on the heat 

 evolved in muscular activity, and from this time he exchanged 

 ideas regularly with du Bois-Reymond, partly in letters written 

 about once a fortnight, partly at meetings between the two 

 friends in Berlin or Potsdam. Up to July i, he was on duty 

 in the field hospital, after which he was 'happy again with 

 leisure to experiment'. After satisfying himself 'with great 

 difficulty as to the constancy of the frog-current between 

 copper electrodes in a solution of copper sulphate', he pro- 

 ceeded to investigate the nature of the chemical processes 

 which he had discovered in muscle. 



At the beginning of October, 1846, Helmholtz sent a ' Report 

 on Work done on the Theory of Animal Heat for 1845', 

 at du Bois' request, to the Fortschritte der Physik, issued 

 by the Physical Society. This was merely an abstract from 

 the article in the Encyclopaedic Dictionary above mentioned, 

 but it anticipates more definitely the conclusions of his great 

 work. He states without hesitation that the material theory 

 of heat is no longer tenable, and that a kinetic theory must 

 be substituted for it, since heat originates in mechanical forces, 

 either directly by friction, or indirectly from an electrical 

 current produced by the motion of magnets. This conception 

 of heat as motion involves the conclusion that mechanical, 

 electrical, and chemical forces must always be the definite 

 equivalent of one and the same energy, whatever the mode 

 by which one force is transformed into another. The empirical 



