practice, the wood ashes were boiled in water, which was then strained
off, and the resulting filtrate, or lye, was mixed with the fats for
soap making.

Most fats contain a substance of an acid nature, and are decomposed by
the action of bases such as caustic soda and caustic potash. The acid
component of the grease partially neutralizes the base, and a new
substance is formed, namely, soap.

With the advance of civilization the labor of soap making passed from
the home to the factory, very much as bread making has done in our own
day. Different varieties of soaps appeared, of which the hard soap was
the most popular, owing to the ease with which it could be
transported. Within the last few years liquid soaps have come into
favor, especially in schools, railroad stations, and other public
places, where a cake of soap would be handled by many persons. By
means of a simple device (Fig. 157), the soap escapes from a
receptacle when needed. The mass of the soap does not come in contact
with the skin, and hence the spread of contagious skin diseases is
lessened.

[Illustration: FIG. 157.--Liquid soap container.]

Commercial soaps are made from a great variety of substances, such as
tallow, lard, castor oil, coconut oil, olive oil, etc.; or in cheaper
soaps, from rosin, cottonseed oil, and waste grease. The fats which go
to waste in our garbage could be made a source of income, not only to
the housewife, but to the city. In Columbus, Ohio, garbage is used as
a source of revenue; the grease from the garbage being sold for soap
making, and the tankage (Section 188) for fertilizer.

206. Why Soap Cleans. The natural oil of the skin catches and
retains dust and dirt, and makes a greasy film over the body. This
cannot be removed by water alone, but if soap is used and a generous
lather is applied to the skin, the dirt is "cut" and passes from the
body into the water. Soap affects a grease film and water very much as
the white of an egg affects oil and water. These two liquids alone do
not mix, the oil remaining separate on the surface of the water; but
if a small quantity of white of egg is added, an emulsion is formed,
the oil separating into minute droplets which spread through the
water. In the same way, soap acts on a grease film, separating it into
minute droplets which leave the skin and spread through the water,
carrying with them the dust and dirt particles. The warmer the water,
the better will be the emulsion, an