40 OF THE ASSIMILATION OF CARBON. 



of undergoing transformation. All the organs to- 

 gether cannot generate a single element, carbon, 

 nitrogen, or a metallic oxide. 



When the quantity of the food is too great, or is 

 not capable of undergoing the necessary transfor- 

 mation, or exerts any peculiar chemical action, the 

 organ itself is subjected to a change : all poisons 

 act in this manner. The most nutritious sub- 

 stances may cause death. In experiments such as 

 those described above, every condition of nutrition 

 should be considered. Besides those matters which 

 form their principal constituent parts, both animals 

 and plants require others, the peculiar functions 

 of which are unknown. These are inorganic sub- 

 stances, 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 metallic compounds. 



If we knew with certainty that there existed a sub- 

 stance capable, alone, of nourishing a plant and of 

 bringing it to maturity, we might be led to a know- 

 ledge of the conditions necessary to the life of all 

 plants, by studying its characters and composition. 

 If humus were such a substance, it would have 

 precisely the same value as the only single food 

 which nature has produced for animal organisation, 

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

 are cheese or caseine, a compound containing nitro- 

 gen in large proportion ; butter, in which hydrogen 

 abounds, and sugar of milk, a substance with a 



