208 SOAP-MAKING. 



added, the mixture being kept constantly heated and 

 well stirred. A cream-like liquid is thus formed, and 

 when the proper quantity of alkali has been added, 

 and the mixture has been boiled long enough, a quan- 

 tity of common salt is stirred in, and the whole is 

 allowed to cool, the fire being withdrawn. 



523. Soap is easily soluble in pure soft water, but 

 it is insoluble, or nearly so, in a strong solution of 

 common salt ; consequently, when salt is added to 

 the newly-formed soap in the boiler, it causes the 

 separation of the soap from the lye. The cream-like 

 liquid quickly separates into two parts, the lower one 

 being clear and transparent, and containing common 

 salt, glycerine, and the impurities of the lye (513), 

 whilst the upper part is thick and opaque. As it 

 cools it solidifies, and forms hard soap. 



524. The addition of common salt to a thick hot 

 solution of soda soap has no other eifect than that just 

 described ; but when it is added to a potash soap, there 

 is produced in addition another effect which is purely 

 chemical. The salts which the fatty acids form with 

 potash are decomposed by the chloride of sodium ; a 

 mutual exchange of acids takes place ; and hence, 

 when a potash soap is mixed with a solution of com- 

 mon salt, both the soap and the chloride of sodium are 

 decomposed, and soda soap and chloride of potassium 

 are the results. 



525. When the spent lye has been separated from 

 the soap, the latter is heated a second time so as to 

 melt it, well stirred up, and then poured into wooden 



