26 INTRODUCTION, 



it was necessary that the fluids should be more abun- 

 dant, and the chemical composition of the animal 

 body more complicated. 



Accordingly azote, an additional substance, enters 

 into the animal composition, as one of its essential 

 elements, while in the plant it is found only acci- 

 dentally united with the other general elements of 

 organization, namely, oxygen, hydrogen, and car- 

 bon ; this peculiarity forms the third of the animal 

 characters. 



Vegetables derive their nutriment from the sun, 

 and from the circumfluent atmosphere in the form 

 of water, which is a combination of oxygen and hy- 

 drogen ; of air containing oxygen and azote ; and of 

 carbonic acid, composed of oxygen and carbon. The 

 peculiar composition of plants requires that the hy- 

 drogen and carbon should be retained, the superflu- 

 ous oxygen exhaled, and a very small quantity, if 

 any, of the azote absorbed. Such accordingly is the 

 process of the vital action in vegetables, where we 

 perceive the most essential functions to be the ex- 

 halation of oxygen, which is performed by the as- 

 sistance of the light. 



The vegetable composition, of which hydrogen 

 and carbon form the principal parts, is also a source 

 of animal nutriment. 



Animals, for the preservation of their own pecu- 

 liar chemical constitution, must get rid of a great 

 portion of the hydrogen and carbon, and absorb and 

 retain more of the azote ; this is done through the 

 medium of the atmosphere, the oxygen of which, 



