ARMY SURGEON AT POTSDAM 47 



opportunity, in discussing the discovery of the Law of the 

 Conservation of Energy, of insisting that it was Mayer who 

 had first expressed his conviction that the sum of energy 

 in the universe could neither be destroyed nor added to, and 

 who formulated this view in his law of ' the equivalence of 

 heat and work '. The English physicist Joule had also under- 

 taken an extensive series of experiments, independent of 

 Mayer, with the object of determining the equivalence between 

 heat and work empirically, and a lively controversy as to the 

 priority of Mayer's work had therefore already been raised 

 by those 'who attach more weight to the collection of data 

 than to, the formulation of general principles '. 



Helmholtz himself, both in his original treatise and in 

 discussing the subject afterwards, used to say that the work 

 which he then undertook was one of pure criticism and 

 arrangement, since its principal aim could only be to test 

 the earlier conclusions derived from inductive methods, upon 

 the newly-acquired material. If a law is to hold good through- 

 out the universe for the vast complex of natural processes, 

 it was not in his estimation sufficient merely to state this as 

 Mayer had done ; evidence sufficient to enforce conviction 

 of its probability must be produced, so that scientific men may 

 bear it in mind for future confirmation. 



' In those days it was far more important than might possibly 

 now be the case, to make clear from beginning to end that 

 the law was a law of facts, abstracted from the facts, and to 

 be confirmed again by facts/ 



Helmholtz consistently recognized that when the Law 

 of the Conservation of Energy had made its way later on, 

 and its accuracy was generally accepted, every one would 

 admit that Mayer had in 1842 reached a perception of its 

 meaning and universal significance, just as Faraday must have 

 had a presentiment of the same law long before Joule gave 

 definite scientific expression to it, and filled up the most 

 important gap in the empirical evidence in its favour. 



But Mayer had not arrived at this perception by scientific 

 lethods. After the data (which were familiar enough to many 

 of his predecessors) had arranged themselves in his conscious- 

 less, 'the creative idea presented itself suddenly, not as a 

 lemonstrated truth, but as a problem, for the solution of which 



