THE MECHANISTIC CONCEPTION 7 



mechanical equivalent of heat, according to which a 

 given amount of energy of motion will yield a given 

 amount of heat, and no more. So there arose the 

 conception that the human animal, like all animals 

 and, indeed, all plants, is a mechanical organism 

 directed by the same laws that govern inorganic 

 matter and able to transform energy, as from heat 

 into motion or from motion into heat, though unable 

 to create new energy. At the same time the chemist 

 Liebig confidently put forward the doctrine that 

 oxidations go on in the body, not, as had been sup- 

 posed, in the lungs, where the oxygen is drawn in, 

 but in the tissues, where the oxygen is carried by 

 the blood stream. To-day it is universally recog- 

 nized by physiologists that the body machine con- 

 verts the potential energy of food into two main 

 forms of energy, animal heat and motion, as the 

 steam engine utilizes the energy which comes from 

 the burning of coal or wood. 



Any consideration of the processes of combustion 

 within the living organism brings us face to face with 

 a question of large, general significance : Does the 

 law of the conservation of energy hold good alike 

 in the humanly produced engine and in the animal 

 body? It would be aside from our main purpose 

 to dwell on this highly interesting question. But 

 it is important for us to realize that the many experi- 

 ments which have been made to settle this question 

 justify us in concluding that the law of the conser- 

 vation of energy is equally true for engines of human 



