146 APPLIED SCIENCE 



and then the first portion may be returned to the filter. 

 Filtration cannot be hastened by scraping or stirring the 

 precipitate on the cloth, as this action will merely cause the 

 filtrate to run turbid. 



159. Processes of Purification. When new compounds 

 are manufactured by means of chemical reaction, they are 

 seldom pure. In order to purify the product one or more 

 of the three processes of crystallization, sublimation, and 

 distillation are used. 



160. Crystallization. The crude product obtained 

 directly from a chemical reaction is usually amorphous 

 (not crystalline). To obtain the substance in uniform, 

 well-defined crystals and to separate it from impurities it 

 must be dissolved again with the aid of heat, filtered, and 

 allowed to cool slowly. Then the dissolved substances will 

 separate into large crystals or into very fine crystals termed 

 " crystal meal," according to conditions. Since the large 

 crystals are compact and offer a relatively small surface 

 to the action of water, they dissolve slowly. Crystal meal, 

 on the other hand, dissolves quite readily and is therefore 

 more commonly used. 



The theory of crystallization is based on the fact that 

 every liquid has the power of dissolving substances. This 

 power can usually be increased by raising the temperature 

 of the liquid. There are a few substances, however, whose 

 maximum strength of dissolving is reached at a temperature 

 much lower than the boiling point. When a solution has 

 dissolved all the solid that it can take up, it is said to be 

 saturated; any decrease in the temperature will then result 

 in the separation of a part from the main body of the sub- 

 stance usually as crystals. While crystals are being formed, 



