THE PRINCIPLES OF ORGANIC CHEMISTRY 49 



gives a symbolic representation of the relation of the atoms in 

 the molecule one to another, is known as a constitutional 

 formula. That these formulae do, as a matter of fact, bear 

 some relation to an actual reality in nature, is shown by the 

 circumstance that, once a constitutional formula has been 

 correctly established, the artificial production of the sub- 

 stance is generally only a matter of time. Thus, to take the 

 case of benzene itself, its formula suggests that if three mole- 

 cules of acetylene C 2 H 2 could be induced to combine, benzene 

 C 6 H 6 would result. On passing acetylene through a red-hot 

 tube benzene is actually produced, the reaction being repre- 

 sented as follows : 



CH 



CH 



III = 

 HC CH 

 % 

 CH 



It goes without saying that before any conclusion can 

 be drawn as to the composition or constitution of a com- 

 pound, it is essential that it should be obtained pure. The 

 methods in use in organic chemistry for obtaining compounds 

 in the pure state resolve themselves into crystallisation and 

 distillation. 



Crystallisation is effected by evaporating a solution of 

 the substance in suitable solvents either at the ordinary 

 atmospheric pressure or in vacuo. The crystals first de- 

 posited are usually the purest ; by redissolving these and re- 

 peating the process pure crystals are eventually obtained. This 

 process is known as fractional crystallisation. Crystallisation 

 is often brought about by combining the substance to be 

 purified with some other body with which it will form a 

 crystallisable compound. A notable instance of this method 

 is the case of many of the sugars, which by themselves form 



