ON THE INTERACTION OF NATURAL FORCES. 169 



cess 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 com- 

 mon to all. P'urther, 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 im- 

 portant 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 Cen- 

 tigrade thermometer, corresponds to a mechanical force 

 by which a pound weight would be raised to the height 

 of 1,350 feet: we name this quantity the mechanical 

 equivalent of heat. I may mention here that these facts 

 conduct of necessity to the conclusion, that heat is not, as 

 was formerly imagined, a fine imponderable substance, 

 but that, like light, it is a peculiar shivering motion of 

 the ultimate particles of bodies. In collision and friction, 

 according to this manner of viewing the subject, the mo- 

 tion of the mass of a body which is apparently lost is con- 

 verted into a motion of the ultimate particles of the 

 body ; and conversely, when mechanical force is generated 

 by heat, the motion of the ultimate particles is converted 

 into a motion of the mass. 



Chemical combinations generate heat, and the quantity 



