VITAL METAMORPHOSES. 161 



on by the same re-agents. Yet we cannot, from 

 the similarity of the products, conclude that these 

 two compounds have a similar constitution. In like 

 manner the nature of the products formed by the 

 action of acids on choleic acid does not entitle us 

 to draw any conclusion as to the form in which its 

 elements are united together. 



63. If the problem to be solved by organic che- 

 mistry be this, namely, to explain the changes which 

 the food undergoes in the animal body ; then it is 

 the business of this science to ascertain what ele- 

 ments must be added, what elements must be se- 

 parated, in order to effect, or, in general, to ren- 

 der possible, the conversion of a given compound 

 into a second or a third; but we cannot expect 

 from it the synthetic proof of the accuracy of the 

 views entertained, because every thing in the orga- 

 nism goes on under the influence of the vital force, 

 an immaterial agency, which the chemist cannot 

 employ at will. 



The study of the phenomena which accompany 

 the metamorphoses of the food in the organism, the 

 discovery of the share which the atmosphere or the ele- 

 ments of water take in these changes, lead at once 

 to the conditions which must be united in order to 

 the production of a secretion or of an organized part. 



64. The presence of free muriatic acid in the 

 stomach, and that of soda in the blood, prove beyond 

 all doubt the necessity of common salt for the or- 

 ganic processes ; but the quantities of soda required 



