“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 begin 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 vapour 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 vapour 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 the fireplace, after all?” asked Mr. Bagges. “Not altogether, according to those who are supposed to know better. They are of opinion, that some of the oxygen unites with the carbon and hydrogen of the blood in the lungs; but that most of it is merely absorbed by the blood, and dissolved in it in the first instance.” “Oxygen absorbed by the blood? That seems odd,” remarked Mr. Bagges, “How can that be?”