that, it is only a theory. Carbon is another element; pure charcoal
is carbon. The part of the air that we use when we breathe or when we
burn things is called _oxygen_. Oxygen is an element; it is not made
of anything but itself. There is another gas which is often used to
fill balloons that are to go very high; it is the lightest in the
world and is called _hydrogen_. Hydrogen is an element.

For a long time people thought that water was an element. Water
certainly looks and seems as if it were made only of itself. Yet
during the thousands of years that people believed water was an
element, they were daily putting two elements together and making
water out of them. When you put a kettle, or anything cold, over a
fire, tiny drops of water always form on it. These are not drops of
water that were dissolved in the air, and that condense on the sides
of the cold kettle; if they were, they would gather on the kettle
better in the open air than over the hot fire. Really there is some
of that very light gas, hydrogen, in the wood or coal or gas that
you use, and this hydrogen joins the oxygen in the air to make water
whenever we burn ordinary fuel.

But the best way to prove that water is made of two gases is to take
the water apart and get the gases from it. Here are the directions for
doing this:

EXPERIMENT 90. A regular bought electrolysis apparatus may be
used, or you can make a simple one as follows:

Use a tumbler and two test tubes. If the test tubes are rather
small (3/8'' X 3'') they will fill more quickly. Dissolve a
little lye (about 1/8 teaspoonful) in half a pint of water
to make the water conduct electricity easily, or you may use
sulfuric acid in place of lye. Pour half of this solution
into the tumbler. Pour as much more as possible into the test
tubes, filling both tubes brim full. Cover the mouth of each
test tube with a small square of dry paper or cardboard, and
turn it upside down, lowering it into the tumbler.

The "electrodes" are two 3/4'' pieces of platinum wire (#30),
which are soldered to two pieces of insulated copper wire,
each about 2 feet long.[8] The other ends of the copper wire
are bare. Fasten the bare end of one copper wire to one nail
of the nail plug if you have direct current (d. c.) in the
laboratory, and fasten the bare end of the other wire to the
other nail; then turn on the electricity. If you do not h