not blow the candle out. I merely let the air which I expired pass into
the aperture, and the result was that the light went out for want of
oxygen, and for no other reason. Something or other--namely, my lungs--had
taken away the oxygen from the air, and there was no more to supply the
combustion of the candle. It is, I think, very pretty to see the time it
takes before the bad air which I throw into this part of the apparatus has
reached the candle. The candle at first goes on burning, but so soon as
the air has had time to reach it, it goes out. And, now, I will shew you
another experiment, because this is an important part of our philosophy.
Here is a jar which contains fresh air, as you can see by the circumstance
of a candle or gas-light burning it. I make it close for a little time,
and by means of a pipe I get my mouth over it so that I can inhale the
air. By putting it over water, in the way that you see, I am able to draw
up this air (supposing the cork to be quite tight), take it into my lungs,
and throw it back into the jar.

[Illustration: Fig. 33.]

We can then examine it, and see the result. You observe, I first take up
the air, and then throw it back, as is evident from the ascent and descent
of the water; and now, by putting a taper into the air, you will see the
state in which it is, by the light being extinguished. Even one
inspiration, you see, has completely spoiled this air, so that it is no
use my trying to breathe it a second time. Now, you understand the ground
of the impropriety of many of the arrangements among the houses of the
poorer classes, by which the air is breathed over and over again, for the
want of a supply, by means of proper ventilation, sufficient to produce a
good result. You see how bad the air becomes by a single breathing; so
that you can easily understand how essential fresh air is to us.