60 OF THE ASSIMILATION OF CARBON. 



require others, the peculiar functions of which are 

 unknown. These are inorganic substances, such as 

 common salt, the total want of which is in animals 

 inevitably productive of death. Plants, for the same 

 reason, cannot live unless supplied with certain me* 

 tallic compounds. 



If we knew with certainty that there existed a 

 substance capable, alone, of nourishing a plant and 

 of bringing it to maturity, we might be led to a 

 knowledge of the conditions necessary to the life of 

 all plants, by studying its characters and composi- 

 tion. If humus were such a substance, it would 

 have precisely the same value as the only single food 

 which nature has produced for animal organization, 

 namely, milk. (Prout.) The constituents of milk are 

 cheese or caseine, a compound containing nitrogen 

 in large proportion ; butter, in which hydrogen 

 abounds ; and sugar of milk, a substance with a 

 large quantity of hydrogen and oxygen in the same 

 proportion as in water. It also contains in solution, 

 lactate of soda, phosphate of lime, and common salt ; 

 and a peculiar aromatic product exists in the butter, 

 called butyric acid. The knowledge of the compo- 

 sition of milk is a key to the conditions necessary 

 for the purposes of nutrition of all animals. 



All substances which are adequate to the nourish- 

 ment of animals contain those materials united, 

 though not always in the same form; nor can any 

 one be wanting for a certain space of time, without 

 a marked effect on the health being produced. The 

 employment of a substance as food presupposes a 

 knowledge of its capacity of assimilation, and of the 

 conditions under which this takes place. 



A carnivorous animal dies in the vacuum of an 

 air-pump, even though supplied with a superabun- 

 dance of food ; it dies in the air, if the demands of 

 its stomach are not satisfied; and it dies in pure 

 oxygen gas, however lavishly nourishment be given 

 to it. Is it hence to be concluded, that neither flesh, 



