44 HERMANN VON HELMHOLTZ 



merely give out in other forms what is communicated to them 

 from the store of energy in the universe. 



Yet it still remained an open question whether perpetual 

 motion might not be possible in the great field of the other 

 natural forces, which cannot be reckoned as purely motive 

 heat, electricity, magnetism, light, chemical affinity but are 

 all, nevertheless, in manifold relations with the mechanical 

 processes, since in almost every natural process mechanical 

 effects are produced, and mechanical work is performed. 



Helmholtz's medical studies, and his knowledge of the 

 biological side of natural phenomena, led him in the first 

 instance to consider the possibility of perpetual motion in these 

 processes, which he had studied since 1841. 



After his physiological observations had led him time after 

 time to the conclusion that there could be no perpetuum mobile 

 for the natural forces that here come under consideration, and 

 when he had convinced himself that such a thing was alto- 

 gether impossible, he inverted the problem hitherto pro- 

 pounded by the scientists, as to how the relations between 

 natural energies could be utilized to construct a perpetuum 

 mobile, and asked himself what the relations between the 

 forces of nature must be, if perpetual motion were indeed 

 impossible. 



As a matter of fact, this reversal of the problem had been 

 previously made for heat, though in less general terms, and 

 with less conscious intention, by Robert Mayer and Colding, 

 with whose investigations Helmholtz was not acquainted, and 

 by Joule, whose experiments he heard of first when his own 

 work was completed. Helmholtz found that all known rela- 

 tions of forces lead to the conclusion that perpetual motion 

 is impossible : he plotted out a further series as yet unknown, 

 the actual existence of which had to be ascertained, and 

 endeavoured to formulate all the relations between the dif- 

 ferent processes of nature which could be deduced from his 

 assumption. The result proved that there is no cycle 

 throughout the entire range of natural processes by which 

 mechanical energy can be generated without corresponding 

 expenditure ; the quantity of working force may indeed be 

 lost to the particular machine, but not to the universe as 

 a whole. ' Nature as a whole possesses a store of energy 



