500 Prof. Helmholtz on the Interaction of Natural Forces. 



wanting when the first theoretic notions were published, have 

 since been confirmed by experiment, particularly by those of 

 Joule; and during the last year the most eminent physicist 

 of France, Regnault, has adopted the new mode of regarding the 

 question, and by fresh investigations on the specific heat of gases 

 has contributed much to its support. For some important con- 

 sequences the experimental proof is still wanting, but the num- 

 ber of confirmations is so predominant, that I have not deemed 

 it too early to bring the subject before even a non-scientific 

 audience. 



How the question has been decided you may already infer 

 from what has been stated. In the series of natural processes 

 there is no circuit to be found, by which mechanical force can 

 be gained without a corresponding consumption. The perpetual 

 motion remains impossible. Our reflections, however, gain thereby 

 a higher interest. 



We have thus far regarded the development of force by natural 

 processes, only in its relation to its usefulness to man, as me- 

 chanical force. You now see that we have arrived at a general 

 law, which holds good wholly independent of the application 

 which man makes of natural forces ; we must therefore make 

 the expression of our law correspond to this more general signi- 

 ficance. It is in the first place clear, that the work which, by 

 any natural process whatever, is performed under favourable 

 conditions by a machine, and which may be measured in the 

 way already indicated, may be used as a measure of force 

 common to all. Further, the important question arises. If the 

 quantity of force cannot be augmented except by corresponding 

 consumption, can it be diminished or lost ? For the purposes of 

 our machines it certainly can, if we neglect the opportunity to 

 convert natural processes to use, but as investigation has proved, 

 not for nature as a whole. 



In the collision and friction of bodies against each other, the 

 mechanics of former years assumed simply that living force was 

 lost. But I have already stated that each collision and each act 

 of friction generates heat ; and, moreover, Joule has established 

 by experiment the important law, that for every foot-pound of 

 force which is lost a definite quantity of heat is always gene- 

 rated, and that when work is performed by the consumption of 

 heat, for each foot-pound thus gained a definite quantity of heat 

 disappears. The quantity of heat necessary to raise the tempera- 

 tv\re of a pound of water a degree of the Centigrade thermometer, 

 corresponds to a mechanical force by which a pound weight 

 would be raised to the height of 1350 feet : we name this quan- 

 tity the mechanical equivalent of heat. I may mention here 

 that these facts conduct of necessity to the conclusion, that heat 



