200 H. HelmhoUz on the Interaction of Natural Forces. 



ditions by a macliine, and whicli may be measured in the way 

 already indicated, may be used as a measure of force common to 

 all. Further, the imporhmt question arises, if the quantity of 

 force cannot be augmented except by corresponding consump- 

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

 machines it certainly can, if we neglect the opportunity to con- 

 vert 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 alvVaA's generated, 

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

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

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

 ture of a pound of water a degree of the Centigrade thermoneter, 

 corresponds to a mechanical force by which a pound weight 

 would be raised to the height of 1350 feet: w^e name this quan- 

 tity the mechanical equivalent of heat. I may mention here 

 that these facts conduct of necessity to the conclusionj that heat 

 3S not, as was formerly imagined, a fine imponderable substance, 

 but that, like light, it is a peculiar vibratory 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 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 eflPected, provided that other actions 

 are not at the same time brought into play. If, liowever, me- 

 chanical w^ork 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 8080 lbs. 

 of water one degree of the Centigrade thermometer; from this ij 



we can calculate that the magnitude of the chemical force of 

 attraction between the particles of a pound of coal and the qnnn- 

 tity of oxygen that corresponds to it, is cnpable of lifting a 

 weight of'^lOO pounds to a height of twenty miles. Unfortu- 

 nately in our steam-engines we have hitherto been able to gain 

 only the smallest portion of this work, the greater part being lost 

 in the shape of heat The best expansive engines give back as 



