210 USE OF SOAP. 



529. Soap forms a clear solution in pure water; it 

 ■will not dissolve in saline solutions, and is decomposed 

 bj a great many salts of lime, and, indeed, nearly 

 all earthy and metallic salts decompose soap, and 

 throw down insoluble compounds of the fatty acids 

 with the bases of these salts. It is for this reason that 

 hard water is not fit for washing ; the soap is decom- 

 posed by it, and rendered insoluble. The slimy scum 

 which is formed by soap in such water, is a compound 

 of the fatty acids of the soap, with lime. 



530. Water which is hard in consequence of its 

 containing carbonate of lime (230), deposits earthy 

 matter, and becomes softer by mere exposure to the 

 air ; the same effect is at once produced by boiling 

 the water for a few minutes, so as to drive off the 

 carbonic acid. Water which is hard from the pre- 

 sence of other salts of lime, may be improved by the 

 addition of a little carbonate of soda ; this will throw 

 down some of the lime ; and, rendering the water softer, 

 will make it much fitter for washing, and prevent a 

 great waste of soap. 



531. Soap is easily soluble in spirit of wine; such 

 a solution afi'ords a very good test of the relative 

 hardness of waters ; the white precipitate it forpas, 

 is a measure of the quantity of earthy matter present 

 in the water. When such a solution of soap is eva- 

 porated, the spirit is driven off, and the soap is ob- 

 tained in a transparent state; various perfumed soaps 

 are thus made. A solution of soap in spirit, to which 



