ON THE INTERACTION OF NATURAL FORCES. 183 



consumed material how much heat, or its equivalent 

 work, is thereby generated in an animal body. Unfor- 

 tunately, the difficulty of the experiments is still very 

 great ; but within those limits of accuracy which have 

 been as yet attainable, the experiments show that the 

 heat generated in the animal body corresponds to the 

 amount which would be generated by the chemical pro- 

 cesses. The animal body therefore does not differ from 

 the steam-engine as regards the manner in which it 

 obtains heat and force, but does differ from it in tlie 

 manner in which 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 mate- 

 rials 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 albu- 

 minous substances which 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 super- 

 seded 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 ? The answer 

 is, from the vegetable kingdom ; for only the material 

 of plants, or the flesh of herbivorous animals, can be 



