for a few seconds at the beginning of the
experiment were caused by the air in the bottle being heated and
expanded by the flame. Soon, however, the oxygen in the air was used
so fast that it made up for this expansion, and the bubbles stopped
going out. When practically all the oxygen was used, the flame went
out.

The candle is made mostly of a combination of hydrogen and carbon. The
hydrogen combines with part of the oxygen in the air that is in the
bottle to form a little water. The carbon combines with the rest of
the oxygen to make carbon dioxid, much of which dissolves in the water
below. So there is practically empty space in the bottle where the
oxygen was, and the air outside forces the water up into this space.
The rest of the bottle is filled with the nitrogen that was in the air
and that has remained unchanged.

About how much of the air was oxygen is indicated by the space that
the water filled after the oxygen was combined with the candle.

[Illustration: FIG. 167. The water rises in the bottle after the
burning candle uses up the oxygen.]

CARBON AND HYDROGEN THE CHIEF ELEMENTS IN FUEL. Carbon and hydrogen
make up the larger part of almost every substance that is used for
fuel, including gas, gasoline, wood, and soft coal; alcohol, crude
oil, kerosene, paper, peat, and the acetylene used in automobile and
bicycle lamps. Hard coal, coke, and charcoal are, however, chiefly
plain carbon. Since burning is simply the combining of things with
oxygen, it is plain that when the carbon of fuel joins oxygen we shall
get carbon dioxid (CO_2). When the hydrogen in the fuel joins oxygen,
what must we get?

When things do not burn up completely, the carbon may be left behind
as charcoal. That is what happens when food "burns" on the stove.
But if anything burns up entirely, the carbon or charcoal burns too,
passing off as the invisible gas, carbon dioxid, just as the hydrogen
burns to form steam or water.

It is because almost every fuel forms water when it burns, that we
find drops of water gathering on the outside of a cold kettle or cold
flatiron if either is put directly over a flame. The hydrogen in the
fuel combines with the oxygen of the air to form steam. As the steam
strikes the cold kettle or iron, it condenses and forms drops of
water.

NOTHING EVER DESTROYED. One important result of the discovery that
burning is only a combining of oxygen with the fuel was that people
began to see that nothing i