lements; but so far
the elements we know show no signs of being made of anything except
themselves.

The last section of this book will explain something about the way
the chemist goes to work to find out what elements are hidden in
compounds.

[Illustration: FIG. 161. Water can be separated into two gases by a
current of electricity.]

THE QUICK WAY CHEMISTS WRITE ABOUT ELEMENTS. Since everything in the
world is made of a combination or a mixture of elements, chemists have
found it very convenient to make abbreviations for the names of the
elements so that they can quickly write what a thing is made of.
They indicate hydrogen by the letter H. O always means oxygen to the
chemist; C means carbon; and Cl means chlorine, the poison gas so much
used in the World War. The abbreviation stands for the Latin name of
the element instead of for the English name, but they are often almost
alike. The Latin name for the metal sodium, however, is _natrum_, and
chemists always write Na when they mean sodium; this is fortunate,
because S already stands for the element sulfur. Fe means iron (Latin,
_ferrum_). But I stands for the element iodine. (The iodine you use
when you get scratched is the element iodine dissolved in alcohol.) It
is not necessary for you to remember the chemical symbols unless
you mean to become a chemist or unless you read a good deal about
chemistry. But almost every one knows at least that H means hydrogen,
O means oxygen, and C means carbon.

When a chemist wants to show that water is made of hydrogen two
parts and oxygen one part, he writes it very quickly like this: H_2O
(pronounced "H two O"). "H_2O" means to a chemist just as much as
"w-a-t-e-r" means to you; and it means even more, because it tells
that water is made of two parts hydrogen and one part oxygen. If a
chemist wanted to write, "You can take water apart and it will give
you two parts of hydrogen and also one part of oxygen," this is what
he would put down:

H_2O -> 2H+O.

If he wanted to show that you could combine two parts of hydrogen and
one part of oxygen to form water, he would write it quickly like this:

2H+O -> H_2O.

These are called _chemical equations_. You do not need to remember
them; they are put here merely so that you will know what they look
like. Some of them are much longer and more complicated, like this:

HC_2H_3O_2+NaHCO_3 -> H_2O+CO_2+NaC_2H_3O_2.

This is the chemist's way of saying, "Vinegar is made