Take another instance, which is only too common:  If you are in a crowded room, with plenty of fire and lights and company, doors and windows all shut tight, how often you feel faint—­so faint, that you may require smelling-salts or some other stimulant.  The cause of your faintness is just the same as that of the mouse’s fainting in the box:  you and your friends, and, as I shall show you presently, the fire and the candles likewise, having been all breathing each other’s breaths, over and over again, till the air has become unfit to support life.  You are doing your best to enact over again the Highland tragedy, of which Sir James Simpson tells in his lectures to the working-classes of Edinburgh, when at a Christmas meeting thirty-six persons danced all night in a small room with a low ceiling, keeping the doors and windows shut.  The atmosphere of the room was noxious beyond description; and the effect was, that seven of the party were soon after seized with typhus fever, of which two died.  You are inflicting on yourselves the torments of the poor dog, who is kept at the Grotto del Cane, near Naples, to be stupified, for the amusement of visitors, by the carbonic acid gas of the Grotto, and brought to life again by being dragged into the fresh air; nay, you are inflicting upon yourselves the torments of the famous Black Hole of Calcutta; and, if there was no chimney in the room, by which some fresh air could enter, the candles would soon burn blue—­as they do, you know, when ghosts appear; your brains become disturbed; and you yourselves run the risk of becoming ghosts, and the candles of actually going out.

Of this last fact there is no doubt; for if, instead of putting a mouse into the box, you will put a lighted candle, and breathe into the tube, as before, however gently, you will in a short time put the candle out.

Now, how is this?  First, what is the difference 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.