put the candle out.

Now, how is this? First, what is the difference between the
breath you take in and the breath you give out? And next, why has
it a similar effect on animal life and a lighted candle?

The difference is this. The breath which you take in is, or ought
to be, pure air, composed, on the whole, of oxygen and nitrogen,
with a minute portion of carbonic acid.

The breath which you give out is an impure air, to which has been
added, among other matters which will not support life, an excess
of carbonic acid.

That this is the fact you can prove for yourselves by a simple
experiment. Get a little lime-water at the chemist's, and breathe
into it through a glass tube; your breath will at once make the
lime-water milky. The carbonic acid of your breath has laid hold
of the lime, and made it visible as white carbonate of lime--in
plain English, as common chalk.

Now I do not wish, as I said, to load your memories with
scientific terms: but I beseech you to remember at least these
two, oxygen gas and carbonic acid gas; and to remember that, as
surely as oxygen feeds the fire of life, so surely does carbonic
acid put it out.

I say, "the fire of life." In that expression lies the answer to
our second question: Why does our breath produce a similar effect
upon the mouse and the lighted candle? Every one of us is, as it