ON THE INTERACTION OF NATURAL FORCES. 151 



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 estab- 

 lished 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 consump- 

 tion 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 1,350 feet : we 

 name this quantity the mechanical equivalent of heat. I may 

 mention here that these facts conduct of necessity to the conclu- 

 sion, that heat is not, as was formerly imagined, a fine impon- 

 derable 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 

 motion of the mass of a body which is apparently lost is converted 

 into a motion of the ultimate particles of the body ; and con- 

 versely, 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 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 temperature 

 of 8,086 pounds of water one degree of the Centigrade ther- 

 mometer ; from this we can calculate that the magnitude of the 

 chemical force of attraction between the particles of a pound 

 of coal and the quantity of oxygen that corresponds to it, is 

 capable of lifting a weight of 100 pounds to a height of twenty 

 miles. Unfortunately , in our steam-engines we have hitherto 



