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and electricity — are now known to be peculiar kinds 

 of motion among the imperceptible molecules of which 

 perceptible bodies are composed. The discovery of 

 the " correlation of forces " was the discovery of the 

 fact that any one of these kinds of molecular motion 

 is constantly liable to be transformed into any one of 

 the other kinds, or, now and then, into the molar 

 motion of a perceptible body. Heat is all the time 

 being converted into light, or into electricity, or into 

 the peculiar kind of undulatory motion known as 

 " nerve-force " — and vice versa. And the law of the 

 correlation is that, when any one of these species of 

 motion appears, an equivalent amount of some other 

 species disappears in producing it. Throughout the 

 world the sum-total of motion is ever the same, but 

 its distribution into heat-waves, light-waves, nerve- 

 waves, &c., varies from moment to moment. 



Let us now apply these principles to the case of 

 an organism, such as the human body. All of the 

 "force" — i.e., capacity of motion — present at any 

 moment in the human body is derived from the food 

 that we eat and the air that we breathe. As food is 

 turned into oxygenated blood and assimilated with 

 the various tissues of the body — which themselves 

 represent previously-assimilated food — the molecular 

 movements of the food-material become variously 



