e 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 were, a living
fire. Were we not, how could we be always warmer than the air outside
us? There is a process going on perpetually in each of us, similar to
that by which coals are burnt in the fire, oil in a lamp, wax in a
candle, and the earth itself in a volcano. To keep each of those fires
alight, oxygen is needed; and the products of combustion, as they are
called, are more or less the same in each case--carbonic acid and steam.

These facts justify the expression I just made use of--which may have
seemed to some of you fantastical--that the fire and the candles in the
crowded room were breathing the same breath as you were. It is but too
true. An average fire in the grate requires, to keep it burning, as much
oxygen as several human beings do; each candle or lamp must have its
share of oxygen likewise, and that a very considerable one; and an
average gas-burner--pray attend to this, you who live in rooms lighted
with gas--consumes as much oxygen as several candles. All alike are
making carbonic acid. The carbonic acid of the fire happily escapes up
the chimney in the smoke: but the carbonic acid from the human beings and
the candles remains