goods
you buy are pure silk or wool, or whether there is a cotton
thread mixed with them. Drop one end of a long hair into the
hot lye solution. What happens to it? Drop a speck of meat or
a piece of finger nail into it.

From this experiment you can readily see why lye will burn your skin
and ruin your clothes. You can also see how it softens the food that
sticks to the bottom of the cooking pan and makes the pan easy to
clean. Lye is one of the strongest bases or alkalies in the world.

[Illustration: FIG. 184. The lye has changed the wool cloth to a
jelly.]

HOW SOAP IS MADE. When lye and grease are boiled together, they form
soap. You cannot very well make soap in the laboratory now, as the
measurements must be exact and you need a good deal of strong lye to
make it in a quantity large enough to use. But the fact that soap is
made with oil, fat, or grease boiled with lye, or caustic soda, which
is almost the same thing, shows why a soap must be 99-44/100% pure,
or something like that, if it is not to injure "the most delicate
fabric." If a little too much lye is used there will be free alkali
in the soap, and it will make your hands harsh and sore and spoil the
clothes you are washing. A "pure" soap is one with no free alkali
in it. A "strong" soap is one that does have some free alkali in it;
there is a little too much lye for the oil or fat, so some lye is left
uncombined when the soap is made. This free alkali cleans things well,
but it injures hands and clothes.

When the drainpipe of a kitchen sink is stopped up, you can often
clear it by sprinkling lye down it, and then adding boiling water.
_If you ever do this, stand well back so that no lye will spatter into
your face; it sputters when the boiling water strikes it._ The grease
in the drainpipe combines with the lye when the hot water comes
down; then the soap that is formed is carried down the pipe, partly
dissolved by the hot water.

When you sponge a grease spot with ammonia, the same sort of chemical
action takes place. The ammonia is a base; it combines with the grease
to form soap, and this soap rinses out of the cloth.

THE LITMUS TEST. To tell what things are bases and what are acids,
a piece of paper dyed with litmus is ordinarily used. Litmus is made
from a plant (lichen). This paper is called _litmus paper_. Try the
following experiment with litmus paper:

EXPERIMENT 109. Pour a few drops of ammonia, a base, into a