222 MAWS CONQUEST OF SUBSTANCES 



domestic servant ; diluted sufficiently, it is invaluable in the 

 washing of delicate fabrics and in the removing of stains, and 

 in a more concentrated form it is helpful as a smelling salt 

 in cases of fainting. 



Some concentrated bases are so powerful in their action on 

 grease, cloth, and metal that they have received the designa- 

 tion caustic, and are ordinarily known as caustic soda, caustic 

 potash (lye), and caustic lime. These more active bases are 

 generally called alkalies in distinction from the less active ones. 



204. Neutral Substances. To any acid solution add grad- 

 ually a small quantity of a base, and test the mixture from 

 time to time with blue litmus paper ; at first the paper will turn 

 red quickly, but as more and more of the base is added to the 

 solution, it has less and less effect on the blue litmus paper, 

 and finally a point is reached when a fresh strip of blue paper 

 will not be affected. Such a result indicates infallibly the 

 absence of any acid qualities in the solution. If now red litmus 

 paper is tested in the same solution, its color also will remain 

 unchanged ; such a result indicates infallibly the absence of 

 any basic quality. The solution has the characteristic property 

 of neither acid nor base and is said to be neutral. 



If to the neutral solution an extra portion of base is added, 

 so that there is an excess of base over acid, the neutralization 

 is overbalanced and the red paper turns blue. If to the 

 neutral solution an extra portion of acid is added, so that 

 there is an excess of acid over base, the neutralization is 

 overbalanced in the opposite direction, and the solution ac- 

 quires acid characteristics. 



Most acids and bases will eat and corrode and discolor, while 

 neutral substances will not ; it is for this reason that soap, a 

 slightly alkaline substance, is the safest cleansing agent for 

 laundry, bath, and general work. Good soaps, being care- 

 fully made, are so nearly neutral that they will not fade the 



