rtain portion of air; and then, if we take the atmosphere off
from the outside of it, just as in these cases we put the pressure on--if
we take the pressure off, you will see how it will then go on expanding
and expanding, larger and larger, until it will fill the whole of this
bell-jar, shewing you that wonderful property of the air, its elasticity,
its compressibility, and expansibility, to an exceedingly large extent,
and which is very essential for the purposes and services it performs in
the economy of creation.

We will now turn to another very important part of our subject,
remembering that we have examined the candle in its burning, and have
found that it gives rise to various products. We have the products, you
know, of soot, of water, and of something else which you have not yet
examined. We have collected the water, but have allowed the other things
to go into the air. Let us now examine some of these other products.

Here is an experiment which I think will help you in part in this way. We
will put our candle there, and place over it a chimney, thus. I think my
candle will go on burning, because the air-passage is open at the bottom
and the top. In the first place, you see the moisture appearing--that you
know about. It is water produced from the candle by the action of the air
upon its hydrogen. But, besides that, something is going out at the top:
it is not moisture--it is not water--it is not condensible; and yet, after
all, it has very singular properties. You will find that the air coming
out of the top of our chimney is nearly sufficient to blow the light out I
am holding to it; and if I put the light fairly opposed to the current, it
will blow it quite out. You will say that is as it should be; and I am
supposing that you think it ought to do so, because the nitrogen does not
support combustion, and ought to put the candle out, since the candle will
not burn in nitrogen.

[Illustration: Fig. 29.]

But is there nothing else there than nitrogen? I must now anticipate--that
is to say, I must use my own knowledge to supply you with the means that
we adopt for the purpose of ascertaining these things, and examining such
gases as these. I will take an empty bottle--here is one--and if I hold it
over this chimney, I shall get the combustion of the candle below sending
its results into the bottle above; and we shall soon find that this bottle
contains, not merely an air that is bad as regards the combustion