so long as we confine ourselves within the laws that
Nature has made for our guidance.

LECTURE V.

OXYGEN PRESENT IN THE AIR--NATURE OF THE ATMOSPHERE--ITS PROPERTIES--OTHER
PRODUCTS FROM THE CANDLE--CARBONIC ACID--ITS PROPERTIES.

We have now seen that we can produce hydrogen and oxygen from the water
that we obtained from the candle. Hydrogen, you know, comes from the
candle, and oxygen, you believe, comes from the air. But then you have a
right to ask me, "How is it that the air and the oxygen do not equally
well burn the candle?" If you remember what happened when I put a jar of
oxygen over a piece of candle, you recollect there was a very different
kind of combustion to that which took place in the air. Now, why is this?
It is a very important question, and one I shall endeavour to make you
understand: it relates most intimately to the nature of the atmosphere,
and is most important to us.

We have several tests for oxygen besides the mere burning of bodies. You
have seen a candle burnt in oxygen, or in the air; you have seen
phosphorus burnt in the air, or in oxygen; and you have seen iron-filings
burnt in oxygen. But we have other tests besides these, and I am about to
refer to one or two of them for the purpose of carrying your conviction
and your experience further. Here we have a vessel of oxygen. I will shew
its presence to you: if I take a little spark and put it into that oxygen,
you know, by the experience you gained the last time we met, what will
happen; if I put that spark into the jar, it will tell you whether we have
oxygen here or not. 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-bot