120 HERMANN VON HELMHOLTZ 



principle, among others, that if a magnet is brought from infinity 

 to a body magnetized by induction, mechanical work will be 

 done, the value of which will eventually equal half the potential 

 of the magnetized, in respect of the magnetizing body. But ' in 

 order not to forestall Clausius ', he did not cite all the results 

 which he had already arrived at. At the time when he pub- 

 lished the ' Conservation of Energy ' he had access only to a few 

 isolated portions (apart from their context) of the works of 

 Poisson, Green, and Gauss, and therefore confined himself to 

 the case in which the iron magnetized by induction was per- 

 fectly soft, and so offered no resistance to magnetization (the 

 distribution of the magnetism thus being similar to that of 

 electricity in conductors electrified by induction). It is, how- 

 ever, obvious from a fragmentary note that he had worked 

 out the mathematical aspects of the problems involved, starting 

 with the assumption that the magnetization of any element of a 

 body is proportional to the magnetizing force. 



The memoir thus designed by Helmholtz to refute the 

 attacks of Clausius is of the greatest interest, since on the one 

 hand it gives the first clear indication of the extent and depth 

 of the work already accomplished by Helmholtz in mathe- 

 matics and physics previous to his twenty-fourth year, and on 

 the other it foreshadows the deductions of the marvellous 

 achievements of his later life. 



The most brilliant and the best known of Helmholtz's popular 

 scientific lectures, i.e. that 'On the Interaction of Natural 

 Forces, and recent Physical Discoveries bearing on the same ', 

 was written as the direct consequence of his renewed pre- 

 occupation with the Law of the Conservation of Energy after 

 the appearance of Clausius's criticism, and of the demand that 

 reached him on all sides in Konigsberg for some more popular 

 account of the great principle which was to underlie the 

 science of the future. His stern father's opinion is interesting 

 and characteristic : 



' It has given me the greatest pleasure, partly from its 

 lucidity and wealth of facts, its easy wit, its hold on true 

 science amid all the difficulties of interesting a non-scientific 

 audience, partly from the high ideal relation it establishes 

 between investigations that would otherwise appear totally 

 independent of one another. 



