The interaction of acids and bases may be observed in another way. If blue litmus paper is put into an acid solution, its color changes to red; if now the red litmus paper is dipped into a base solution, caustic soda, for example, its original color is partially restored. What the acid does, the base undoes, either wholly or in part. Bases always turn red litmus paper blue. Bases, like acids, are good or bad according to their use; if they come in contact with cloth, they eat or discolor it, unless neutralized by an acid. But this property of bases, harmful in one way, is put to advantage in the home, where grease is removed from drainpipe and sink by the application of lye, a strong base. If the lye is too concentrated, it will not only eat the grease, but will corrode the metal piping; it is easy, however, to dilute base solutions to such a degree that they will not affect piping, but will remove grease. Dilute ammonia is used in almost every home and is an indispensable 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 designation _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 gradually 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 acquires acid characteristics.