510 Prof. Helmholtz on the Interaction of Natural Forces. 



body, it seems that certain definite albuminous 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 j 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 superseded by coal, simply because the latter does not permit 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 

 ai'ises, 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 ani- 

 mals, 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, per- 

 mit the nourishment to store itself in the animaPs 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 investigation, we find that 

 the principal part of the former consists in the products of com- 

 bustion which are generated by the animal. They take the con- 

 sumed 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 combustible sub- 

 stances, 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. 

 Wovdd not the combination of both organic kingdoms produce 

 the perpetual motion ? We must not conclude hastily : further 

 inquiry shows, that plants are capable of producing combustible 

 substances only when they are under the influence of the sun. 

 A portion of the sun's rays exhibits a remarkable relation to 

 chemical forces, — it can produce and. destroy chemical combina- 

 tions ; and these rays, which for the most part are blue or violet, 



