THE CONSERVATION OF ENERGY 



" In many cases we see motion cease without having 

 caused another motion or the lifting of a weight. But 

 a force once in existence cannot be annihilated it can 

 only change its form. And the question therefore 

 arises, what other forms is force, which we have be- 

 come acquainted with as falling force and motion, 

 capable of assuming ? Experience alone can lead us to 

 a conclusion on this point. That we may experiment 

 to advantage, we must select implements which, be- 

 sides causing a real cessation of motion, are as little as 

 possible altered by the objects to be examined. For 

 example, if we rub together two metal plates, we see 

 motion disappear, and heat, on the other hand, make 

 its appearance, and there remains to be determined only 

 whether motion is the cause of heat. In order to reach 

 a decision on this point, we must discuss the question 

 whether, in the numberless cases in which the expen- 

 diture of motion is accompanied by the appearance of 

 heat, the motion has not some other effect than the 

 production of heat, and the heat some other cause 

 than the motion. 



" A serious attempt to ascertain the effects of ceasing 

 motion has never been made. Without wishing to 

 exclude a priori the hypothesis which it may be possi- 

 ble to establish, therefore, we observe only that, as a 

 rule, this effect cannot be supposed to be an alteration 

 in the state of aggregation of tin moved (that is, 

 rubbing, etc.) bodies. If we assume that a certain 

 quantity of motion v is expended in the conversion of 

 a rubbing substance m into , we must then have 

 m + v n, and n = m + v; and when n is reconvert- 

 ed into m t v must appear again in some form or other. 

 TOL. m. 18 263 



