nitrogen; then the phosphorus used up the oxygen making white
fumes; afterwards, the water sucked up these fumes; and so, in
the jar now nitrogen is the only gas left, and the water has
risen up to fill all the rest of the space that was once taken up
with oxygen.

We can easily prove that there is no oxygen now in the jar. I
take out the cork and let a lighted taper down into the gas. If
there were any oxygen the taper would burn, but you see it goes
out directly proving that all the oxygen has been used up by the
phosphorous. When this experiment is made very accurately, we
find that for every pint of oxygen in air there are four pints of
nitrogen, so that the active oxygen-atoms are scattered about,
floating in the sleepy, inactive nitrogen.

It is these oxygen-atoms which we use up when we breathe. If I
had put a mouse under the bell-jar, instead of the phosphorus,
the water would have risen just the same, because the mouse would
have breathed in the oxygen and used it up in its body, joining
it to carbon and making a bad gas, carbonic acid, which would
also melt in the water, and when all the oxygen was used, the
mouse would have died.

Do you see now how foolish it is to live in rooms that are
closely shut up, or to hide your head under the bedclothes when
you sleep? You use up all the oxygen-atoms, and then there are
none left for you to breathe; and besides this, you send out of
your mouth bad fumes, though you cannot see them, and these, when
you breathe them in again, poison you and make you ill.