54 THE FAIRY-LAND OF SCIENCE. 
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 phosphorus. 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. 
Perhaps you will say, If oxygen is so useful, why is 
not the air made entirely of it ? But think for a 
moment. If there was such an immense quantity of 
oxygen, how fearfully fast everything would burn ! 
Our bodies would soon rise above fever heat from the 
quantity of oxygen we should take in, and all fires and 
