t what makes the blood change its color?"

"Recollect, uncle, you have a little branch from the windpipe opening into
the cell which lets in the air. Then the blood and the air are brought
together, and the blood alters in color. The reason, I suppose you would
guess, is that it is somehow altered by the air."

"No very unreasonable conjecture, I should think," said Mr. Bagges.

"Well; if the air alters the blood, most likely, we should think, it gives
something to the blood. So first let us see what is the difference between
the air we breathe _in_, and the air we breathe _out_. You know that
neither we nor animals can keep breathing the same air over and over
again. You don't want me to remind you of the Black Hole of Calcutta, to
convince you of that; and I dare say you will believe what I tell you,
without waiting till I can catch a mouse and shut it up in an air-tight
jar, and show you how soon the unlucky creature will get uncomfortable,
and began to gasp, and that it will by-and-by die. But if we were to try
this experiment--not having the fear of the Society for the Prevention of
Cruelty to Animals, nor the fear of doing wrong, before our eyes--we should
find that the poor mouse, before he died, had changed the air of his
prison considerably. But it would be just as satisfactory, and much more
humane, if you or I were to breathe in and out of a silk bag or a bladder
till we could stand it no longer, and then collect the air which we had
been breathing in and out. We should find that a jar of such air would put
out a candle. If we shook some lime-water up with it, the lime-water would
turn milky. In short, uncle, we should find that a great part of the air
was carbonic acid, and the rest mostly nitrogen. The air we inhale is
nitrogen and oxygen; the air we exhale has lost most of its oxygen, and
consists of little more than nitrogen and carbonic acid. Together with
this, we breathe out the vapor of water, as I said before. Therefore in
breathing, we give off exactly what a candle does in burning, only not so
fast, after the rate. The carbonic acid we breathe out, shows that carbon
is consumed within our bodies. The watery vapor of the breath is a proof
that hydrogen is so, too. We take in oxygen with the air, and the oxygen
unites with carbon, and makes carbonic acid, and with hydrogen, forms
water."

"Then don't the hydrogen and carbon combine with the oxygen--that is,
burn--in the lungs, and isn't the chest t