F CATION AND PREPARATION OF PURE 

 ORGANIC COMPOUNDS. 



CRITERIA OF THEIR PURITY. 



The organic material which composes all animal and vegetable 

 cells consists of a mixture of numerous compounds. In order to in- 

 vestigate their evolution and their degradation in nature it is necessary 

 to separate these compounds from one another and to prepare each of 

 them in a state of purity. The pure substance can then be analysed 

 and its chemical and physical properties ascertained. Knowledge of 

 the pure compounds shows their chemical relationship to one another 

 and an idea of their role in nature is obtained. This idea is proved or 

 disproved by an investigation of the changes which the organism, as a 

 whole or individual portions of it, can effect in these substances. 



In the study of the chemical properties of the compounds, other 

 compounds are formed by their interaction. These compounds also 

 require isolation and purification. The principal operations in organic 

 and biological chemistry will thus consist in the isolation and prepara- 

 tion of pure compounds. 



The methods of separating organic compounds are based upon 

 differences in the properties of the substances under investigation. 

 These differences are taken advantage of as much as possible ; some- 

 times they are so gross that the separation is simple, sometimes they are 

 so small that the separation is of extreme difficulty, and in these cases 

 a separation can only be effected when sufficient material is available. 



Solid organic compounds are more numerous than liquid ; gases are 

 comparatively rare. 



Solids are most usually separated and purified by solution in water 

 or organic solvents. The methods for the manipulation of liquids are 

 therefore most frequently used and are described first. 



