HELMHOLTZ IN BERLIN 



studying d'Alembert's celebrated and successful 

 enunciation of the principle of active and lost 

 forces ; but, of course, the new principle was not as 

 clear to me from the beginning as it was when I 

 wrote my treatise in 1843. I closed my discussion 

 by showing that the discovery of a perpetuum mobile 

 would be possible if my principle was wrong.' x Here 

 he anticipates Helmholtz. 



The researches of Joule were final and conclusive. 

 Extending over a period of several years, probably 

 originating before 1840 (the first paper appeared in 

 that year), they are models of skilful experiment and 

 accurate induction. They were concluded in 1849, 

 when the dynamical equivalence of heat was finally 

 established. This led to the enunciation of the first 

 law of thermo-dynamics, that c when equal quantities 

 of mechanical effect are produced by any means what- 

 ever from purely thermal sources, or lost in purely 

 thermal effects, then equal quantities of heat are put 

 out of existence or are generated ; and for every unit 

 of heat measured by the raising of a pound of water, 

 i Fahrenheit in temperature, you have to expend 

 772 foot-pounds of work.' The principle was estab- 

 lished by Joule for mechanical work, current electricity, 

 electro - magnetism and light. Thus the grandest 

 generalisation of physical science the conservation of 

 energy is founded on the mechanical theory of heat. 2 



1 Colding, Phil. Mag., Jan. 1864. 



2 Tait, Recent Advances, op. cit., p. 64. 



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