CH. xxxiv. CONSERVATION OF ENERGY. 339 



height of '772 feet. This, you will notice, was exactly the con- 

 verse of Joule's experiment, and proved that exactly as much 

 motion is produced by means of confined heat as there is 

 heat produced by means of checked motion. 



Conservation of Energy. And thus we arrive at one 

 of the grandest discoveries of modern science, namely, that 

 the whole amount of energy ', or power of doing work, pos- 

 sessed by any set of bodies, remains unaltered whatever 

 transformations it may undergo. It may exist in one of 

 two forms either as potential or stored-up energy, which is 

 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 weight which had been pulled up 

 772 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 Him 

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

 can be turned back into enough energy to raise a 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 reproduced exactly the same 

 amount of visible energy or actual work. 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 wherever it disappears 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 motion, 

 or motion into heat, the energy which causes them both 



