Yes! We have proved it by combustion; and now here is
another test for oxygen, which is a very curious and useful one. I have
here two jars full of gas, with a plate between them to prevent their
mixing; I take the plate away, and the gases are creeping one into the
other. "What happens?" say you: "they together produce no such combustion
as was seen in the case of the candle." But see how the presence of oxygen
is told by its association with this other substance[14]. What a
beautifully coloured gas I have obtained in this way, shewing me the
presence of the oxygen! In the same way we can try this experiment by
mixing common air with this test-gas. Here is a jar containing air--such
air as the candle would burn in--and here is a jar or bottle containing
the test-gas. I let them come together over water, and you see the result:
the contents of the test-bottle are flowing into the jar of air, and you
see I obtain exactly the same kind of action as before, and that shews me
that there is oxygen in the air--the very same substance that has been
already obtained by us from the water produced by the candle. But then,
beyond that, how is it that the candle does not burn in air as well as in
oxygen? We will come to that point at once. I have here two jars; they are
filled to the same height with gas, and the appearance to the eye is alike
in both, and I really do not know at present which of these jars contains
oxygen and which contains air, although I know they have previously been
filled with these gases.
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