HERMANN VON HELMHOLTZ 285 



much to its support. For some important consequences the experi- 

 mental proof is still wanting, but the number 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 im- 

 possible. 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 mechanical 

 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 new 

 law correspond to this more general significance. 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, *Tf the quantity of force cannot be augmented except by cor- 

 responding consumption, can it be diminished or lost ?" For the pur- 

 pose of our machines it certainly can, if we neglect the opportunity 

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

 not for a nature as a whole. 



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

 chanics 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 generated, 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 temperature 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 quantity the mechanical equivalent of heat. I may mention here 

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

 is not, a9 was formerly imagined, a fine imponderable substance. 



