184 ox THE INTERACTION OF NATURAL FORCES. 



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, 

 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 

 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 com- 

 bination 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 per- 

 petual store of force. Plants prepare fuel and nutri- 

 ment, 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 : fm'ther inquiry shows, that plants are capable of 

 producing combustible substances only when they are 



