NATURAL PHILOSOPHY. 135 



anmial 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 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 but- 

 ter, just as well as with coal or wood ; the animal body must dissolve its ma- 

 terials artificially, and distribute them through its system ; it must, further 

 perpetually renew the used-up materials of its organs, and as it cannot it- 

 self 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 nutri- 

 ment. As material for the perpetual renewal of the body, it seems that 

 certain definite albuminous substances which appear i.i plants, and from the 

 chief mass of the animal body, can alone be used. They form only a por- 

 tion 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 are really only materials for warming, and are perhaps not to be superseded 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 dis- 

 tinguished 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 

 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 vegetables, which the former could not make use of im- 

 mediately as nutriment. In hay and grass the same nutritive substances 

 are present as in meal and flour, but in less quantity. As, however, the di- 

 gestive 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, therefore, we are referred to the vegetable world. Now when 

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

 gation, we find that the principal part of the former consists in the products 

 of combustion which are generated by the animal. They take the consumed 

 carbon given off in respiration, as carbonic acid, from the air, the consumed 

 hydrogen as water, the nitrogen in its simplest and closest combination as 

 ammonia; and from these materials, with the assistance of small ingredients 

 which they take from the soil, they generate anew the compound combusti- 

 ble substances, albumen, sugar, oil, .on which the animal subsists. Here, 

 therefore, is a circuit which appears to be a perpetual store of force.* Plants 

 prepare fuel and nutriment, animals consume these, burn them slowly in 

 their lungs, and from the products of combustion the plants again derive 

 their nutriment. The latter is an eternal source of chemical, the former of 

 mechanical forces. Would not the combination of both organic kingdoms 

 produce the perpetual motion ? We must not conclude hastily : further in- 

 quiry shows, that plants are capable of producing combustible substances 



