CH. xxxv. CONVERSION OF MOTION INTO HEAT. 359 



unseen by us, or as visible energy, when it is actually per- 

 forming work ; but while it changes from one form to another 

 its amount never alters. Thus in Joule's experiment the 

 energy stored up in the i Ib. weight which had been pulled 

 up 7 7 2 feet was gradually transformed, as soon as the weight 

 was released, into an amount of heat capable of raising the 

 temperature of a pound of water i Fahr. ; while Hirn 

 showed, on the other hand, that exactly this amount of heat 

 would, if it could be turned back again into energy, raise the 

 i Ib. weight to the height of 772 feet at which it stood 

 before. 



The potential energy, or power of doing work, remained, 

 therefore, exactly the same whether it was stored up in the 

 weight or in the hot water. And even though we know 

 that practically some energy disappears at every part of a 

 machine when it is at work, yet this is not lost ; for it turns 

 into heat, or some other form of energy, wherever it disap- 

 pears as motion. If you grease the wheels of a machine, 

 you will detect this heat beginning to do work again by 

 turning the solid grease into a liquid. 



By whatever means, therefore, heat is turned into work, 

 or work into heat, the energy which causes them both 

 remains the same, and this is one out of many proofs that 

 energy cannot be destroyed, but is only lost in one form to 

 reappear in another. 



Although the experiments and calculations which have 

 proved heat to be a mode of motion are some of the most 

 interesting which have been made of late years, yet they are 

 by . no means the only ones. In 1 8 1 1 Sir John Leslie 

 carried on a most interesting series of observations on the 

 reflection of heat : and the Italian physicist Melloni has 

 traced the whole passage of heat-rays through different 

 solid bodies. All these discoveries are clearly and simply 



