238 INTERACTION OF NATUEAL FOKCES. 



the steps in which it occurs, we can calculate from the mass of 

 the consumed material how much heat, or its equivalent work 

 is thereby generated in an animal body. Unfortunately, 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 processes. 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 the man- 

 ner 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 ani- 

 mal 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 perpet- 

 ual renewal of the body, it seems that certain definite albumi- 

 nous 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 takett daily ; the remain- 

 der, 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 and are perhaps not to be superseded by coal, simply because 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 ques- 

 tion 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 plant-eating 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 



