52 THE WONDERFUL CENTURY. CHAP. vn. 



water from the freezing to the boiling point. But, dur- 

 ing the operation, the metal had lost no weight or under- 

 gone any other change ; and as the production of heat by 

 this process appeared to be unlimited, he concluded that 

 heat could not be matter, but merely a kind of motion set 

 up in the particles of matter by the force exerted. 

 Bacon and Locke had expressed similar ideas long be- 

 fore; and, later, Sir Humphrey Davy showed that by 

 rubbing together two pieces of ice at a temperature be- 

 low the freezing point sufficient heat was produced to 

 partially melt them; while other observers found that to 

 shake water in a bottle raised its temperature, and that 

 percussion or compression, as had been long known, pro- 

 duced a considerable amount of heat. These various 

 facts led to the conclusion that there was a mechanical 

 equivalent of heat that is, that a certain amount of 

 force exerted or work done would produce a correspond- 

 ing amount of heat ; and Joule was the first to determine 

 this accurately by a number of ingenious experiments. 

 The result was found to be that a pound of water can be 

 raised 1 C. by an amount of work equal to that required 

 to raise one pound to the height of 1392 feet, or 1392 

 Ibs. one foot. Various experiments with different ma- 

 terials were found always to lead to the same result, and 

 thus the final blow was given to the material theory of 

 heat, which was thenceforth held to be a mode of motion 

 of the molecules of bodies. 



These conclusions led to the more general law of the 

 conservation of energy, which implies that in any limited 

 system of bodies, whether a steam-engine or the solar 

 system, no change can occur in the total amount of the 

 energy it contains unless fresh energy comes to it from 



