ent, the escaping oxygen passed through the tube, filled
the bottle, and forced the water out.

WHAT BURNING IS. When anything burns, it is simply joining oxygen.
When a thing burns in air, it cannot join the oxygen of the air very
fast, for every quart of oxygen in the air is diluted with a gallon of
a gas called _nitrogen_. Nitrogen will not burn and it will not
help anything else to burn. But when you have pure oxygen, as in the
bottle, the particles of wood or charcoal or picture wire can join it
easily; so there is a very bright blaze.

Although free oxygen helps things to burn so brilliantly, a match
applied to the solids from which you got it would go out. And while
hydrogen burns very easily, you cannot burn water although it is
two-thirds hydrogen. Water is H_2O, you remember.

WHAT COMPOUNDS ARE. When elements are combined with other elements,
the new substances that are formed are called _compounds_. Water
(H_2O) is a compound, because it is made of hydrogen and oxygen
combined.

When elements unite to form compounds, they lose their original
qualities. The oxygen in water will not let things burn in it; the
hydrogen in water will not burn. Salt (NaCl) is a compound. It is made
of the soft metal sodium (Na), which when placed on water sputters and
drives hydrogen out of the water, and the poison gas chlorine (Cl),
combined with each other. And salt is neither dangerous to put in
water like sodium, nor is it a greenish poison gas like chlorine.

MIXTURES. But sometimes elements can be mixed without their combining
to form compounds, in such a way that they keep most of their original
properties. Air is a mixture. It is made of oxygen (O) and nitrogen
(N). If they were _combined_, instead of _mixed_, they might form
laughing gas,--the gas dentists use in putting people to sleep when
they pull teeth. So it is well for us that air is only a _mixture_ of
oxygen and nitrogen, and _not_ a compound.

You found that things burned brilliantly in oxygen. Well, things burn
in air too, because a fifth of the air is oxygen and the oxygen of
the air has all its original properties left. Things do not burn as
brightly in air as they do in pure oxygen for the same reason that a
teaspoonful of sugar mixed with 4 teaspoonfuls of boiled rice does not
taste as sweet as pure sugar. The sugar itself is as sweet, but it is
not as concentrated. Likewise the oxygen in the air is as able to help
things burn as pure oxygen is; bu