ere is a more practical experiment showing the action of acid on
metal:

EXPERIMENT 106. Use two knives, one of bright steel and the
other a silver-plated one. If the steel knife is not bright,
scour it until it is. Drop a little lemon juice on each knife
and let it stand for a few minutes, while the teacher does the
next experiment. Then rinse both knives and examine them. What
has the lemon juice done to the silver knife? to the steel
one?

The lemon juice acts in this way because it is acid. Acids act on the
taste nerves in the tongue and give the taste of sourness; everything
sour is an acid. The black stuff formed on the steel knife by the
lemon juice is an iron salt. The iron in the knife drove the hydrogen
out of the lemon juice, but there was not enough for you to see it
coming off; then the iron combined with the rest of the lemon juice to
form an iron salt.

Whenever an acid acts on a metal, the metal drives off the hydrogen
and forms a salt. The salt is not always good to eat; for instance,
the salt that tin forms with acids is poisonous.

ACTION OF ACIDS ON OTHER SUBSTANCES. Acids do not act on metals only,
however. Watch the next experiment to see what a strong acid will do
to cloth.

EXPERIMENT 107. _To be done by the teacher._ Put a drop of
concentrated nitric or sulfuric acid on a piece of colored
wool cloth, or on a piece of colored silk. Let it stand for
a few minutes, then rinse it thoroughly. Test the cloth where
the acid has been to see whether or not it is as strong as the
rest of the cloth. How has the acid affected the color?

[Illustration: FIG. 183. Strong acids will eat holes like this in
cloth.]

ACTION OF ACIDS ON THE NERVES OF TASTE. Acids act on the taste nerves
in the tongue and give the taste of sourness; everything sour is an
acid. Lemon juice, sour milk, and sour fruits are all too weak acids
to injure clothes or skin, but their sour taste is a result of the
acid in them acting on the nerves of taste.

_APPLICATION 79._ A girl wanted to make lemonade. She did
not know which of two knives to use, a steel-bladed one or a
silver-plated one. Which should she have used?

_APPLICATION 80._ A woman was going to put up some tomatoes.
She needed something large to cook them in. She had a shiny
new tin dish pan, an older enamelware dish pan, a galvanized
iron water pail, and an old-fashioned copper kettl