ON THE INTERACTION OF NATURAL FORCES. 163 



the force gained is to be made use of. The body is, besides, 

 more limited than the machine in the choice of its fuel ; the 

 latter could be heated with sugar, with starch-flour, and butter, 

 just as well as with coal or wood ; the animal body must dissolve 

 its materials artificially, and distribute them through its system; 

 it must, further, perpetually renew the used-up materials of its 

 organs, and as it cannot itself create the matter necessary for this, 

 the matter must come from without. Liebig was the first to 

 point out these various uses of the consumed nutriment. As 

 material for the perpetual renewal of the body, it seems that 

 certain definite albuminous substances whieh appear in plants, 

 and form the chief mass of the animal body, can alone be used. 

 They form only a portion of the mass of nutriment taken daily ; 

 the remainder, sugar, starch, fat. are really only materials for 

 warming, and are perhaps not to be superseded by coal, simply 

 because the latter does not permit itself to be dissolved. 



If, then, the processes in the animal body are not in this 

 respect to be distinguished from inorganic processes, the question 

 arises, Whence comes the nutriment which constitutes the source 

 of the body's force 1 The answer is, from the vegetable king- 

 dom for only the material of plants, or the flesh of herbivorous 

 animals, can be made use of for food. The animals which live 

 on plants occupy a mean position between carnivorous animals, 

 in which we reckon man, and vegetables, which the former 

 could not make use of immediately as nutriment. In hay and 

 grass the same nutritive substances are present as in meal and 

 flour, but in less quantity. As, however, the digestive organs 

 of man are not in a condition to extract the small quantity 

 of the useful from the great excess of the insoluble, we submit, 

 in the first place, these substances to the powerful digestion of 

 the ox, permit the nourishment to store itself in the animal's 

 body, in order in the end to gain it for ourselves in a more 

 agreeable and useful form. In answer to our question, there- 

 fore, we are referred to the vegetable world. Now when what 

 plants take in and what they give out are made the subjects of 

 investigation, we find that the principal part of the former 

 consists in the products of combustion which are generated by 



