226 INTERACTION OF NATURAL FOKCES. 



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 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 de- 

 gree of the centigrade thermometer, corresponds to a mechani- 

 cal 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 con- 

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

 was formerly imagined, a fine imponderable substance, but 

 that, like light, it is a peculiar shivering motion of the ulti- 

 mate particles of bodies. In collision and friction, according 

 to this manner of viewing the subject, the motion of the mass 

 of a body which is apparently lost is converted into a motion 

 of the ultimate particles of the body ; and conversely, when 

 mechanical force is generated by heat, the motion of the ulti- 

 mate particles is converted into a motion of the mass. 



Chemical combinations generate heat, and the quantity of 

 this heat is totally independent of the time and steps through 

 which the combination has been effected, provided that other 

 actions are not at the same time brought into play. If, however, 

 mechanical work is at the same time accomplished, as in the 

 case of the steam engine, we obtain as much less heat as is 

 equivalent to this work. The quantity of work produced by 

 chemical force is in general very great. A pound of the 

 purest coal gives, when burnt, sufficient heat to raise the tem- 

 perature of 8086 pounds of water one degree of the centi- 

 grade thermometer ; from this we can calculate that the mag- 

 nitude of the chemical force of attraction between the parti- 



