ENERGY 113 



grade 1 for one gram of the water. In other words, 

 he found that energy, which was apparently lost, was 

 not lost, as had been believed up to that time, but 

 was converted into molecular energy and manifested 

 as heat. He also found the numerical relation between 

 temperature and the molecular energy of water. He 

 is usually said to have determined the " mechanical 

 equivalent of heat." This term, however, is mislead- 

 ing, and we shall discuss it later. 2 



Joule's experiments extended through several years 

 and covered a wide range of possible cases, of which the 

 one described above is merely the simplest for such a 

 discussion as ours. In all cases he found that when 

 energy, which was associated with ponderable bodies, 

 disappeared there was to be measured a perfectly 

 definite amount of increase in heat in the parts of the 

 system with which he was working. Of course, his 

 laboratory methods were not always as precise as they 

 might be made to-day ; and some of them do not permit 

 of very accurate observations. His experiments, how- 

 ever, satisfied the scientific world of the truth of his 

 idea that energy is never destroyed but merely changed 

 in kind or in location. 



This fundamental fact of physical science, which 

 Joule demonstrated is known to-day as the Principle 

 of the Conservation of Energy, but is better expressed 

 as the Principle of the Indestructibility of Energy. 



Energy and matter are both indestructible. Energy, 

 as we have seen, may change its form and its location, 



1 One degree centigrade is 1.8 degrees on the Fahrenheit scale of 

 temperatures. Also C. is the same temperature as 32 F. 



2 Cf. Chapter XXI. 



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