ncentrated 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.

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 carefully made, are so nearly
neutral that they will not fade the color out of clothing; the cheap
soaps are less carefully prepared and are apt to have a strong excess
of the base ingredient; such soaps are not safe for delicate work.

205. Soap. If we gather together scrapings of lard, butter, bits of
tallow from burned-out candles, scraps of waste fat, or any other sort
of grease, and pour a strong solution of lye over the mass, a soft
soapy substance is formed. In colonial times, every family made its
own supply of soap, utilizing, for that purpose, household scraps
often regarded by the housekeeper of to-day as worthless. Grease and
fat were boiled with water and hardwood ashes, which are rich in lye,
and from the mixture came the soft soap used by our ancestors. In