HERMANN VON HELMHOLTZ 



ciple of the conservation of energy to living beings 

 was first clearly made by Robert Mayer. Nothing 

 will give a better notion of this line of thought than 

 the following quotation from Mayer's well-known 

 paper on 4 Organic Motion and Nutrition': 



1 The second question refers to the cause of the 

 chemical tension produced in the plant. This ten- 

 sion is a physical force. It is equivalent to the heat 

 obtained from the combustion of the plant. Does 

 this force, then, come from the vital processes, and 

 without the expenditure of some other form of force ? 

 The creation of a physical force, of itself hardly 

 thinkable, seems all the more paradoxical when we 

 consider that it is only by the help of the sun's rays 

 that plants perform their work. By the assumption 

 of such a hypothetical action of the "vital force," 

 all further investigation is cut off, and the application 

 of the methods of exact science to the phenomena 

 of vitality is rendered impossible. Those who hold 

 a notion so opposed to the spirit of science would 

 be thereby carried into the chaos of unbridled phan- 

 tasy. I therefore hope that I may reckon on the 

 reader's assent when I state, as an axiomatic truth, 

 that during vital processes a conversion only of matter^ 

 as well as of force, occurs^ and that creation of either the 

 one or the other never takes place. 



'The physical force collected by plants becomes 

 the property of another class of creatures of animals. 

 The living animal consumes combustible substances 

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