"In one of our recent lessons we learned why hot air rises, and why the hottest air in the room is always near the ceiling," said Mr. Wilson. "But the burning of the candle, the gas, or the oil in the lamp affects the air of the room in another way besides heating it, and thus causing it to expand and become lighter than it was. Who can tell what I mean?""The burning, in each case, produces the poisonous carbonic acid gas, sir," said Will, "and this gas floats away into the air of the room.""That is exactly what I meant, my lad," said Mr. Wilson. "But I think you can tell of another way by which the air becomes charged with carbonic acid gas, in addition to this.""Yes, sir; we and all other animals are constantly sending out carbonic acid gas into the air by our breathing.""Quite right, Will. Now think again. You called this carbonic acid a poisonous gas just now. Why?""We could not breathe this gas, sir, even in a small quantity, without making ourselves ill; and in a larger quantity it would quickly kill us.""Right again," said Mr. Wilson. "Let me make this quite clear. Imagine a man shut up for twenty-four hours in a room 7 feet square and 8 feet high. At the end of that time every particle of air in the room would have passed through his lungs; he would have robbed the air of aboutone-twentieth of its oxygen, and breathed out into it an equal amount of carbonic acid gas.
"Such air would be quite unfit to breathe. But as long as the man lives he must continue to breathe, and thus at every breath he would be inhaling large quantities of this poisonous gas. We have only to picture two or three people in that room instead of one, and think of what the result would be.
"The very thought of it calls up in one"s mind the hideous story of the Black Hole of Calcutta. On 20th June 1756, Surajah Dowlah, Nabob of Bengal, made a sudden and unexpected attack on the defenceless traders at Calcutta, or Fort William, as it was then more commonly called. After entering the town, he took 146 persons of our race and drove them at the point of the sword into a small room 18 feet long and 14 broad, where they were shut up for the night, the place being surrounded and strongly guarded by the Nabob"s troops. There, through the sultry night of that hot climate, these poor sufferers endured unspeakable agonies of thirst and suffocation, trampling one another down in vain attempts to reach the one little window in the dungeon, filling the air with theirwild ravings of delirium and despair. The dusky guards outside, meanwhile, replied to their appeals with jeers and curses. In the morning, 23 out of the 146, one of them a woman, were dragged from this den of torment alive, but pale, emaciated, exhausted, and quite unable to stand. The remainder were a heap of corpses.
"In our school-rooms, public halls, and all buildings where many people meet, as well as in our living and sleeping rooms at home, every care should be taken to see that there is a way of escape for the heated, vitiated air, and a free communication with the fresh air on the outside.
"This is what we mean by ventilation. There are many artificial contrivances for admitting the fresh air and driving out the bad. Most of them are constructed somewhat on the principle of the fan; they set the air in motion just as a fan does when we move it to and fro.
"The word ventilation itself comes from a Latin word, which actually describes the waving movement of a fan through the air.
"We cannot all have costly contrivances for ventilating our rooms, but we can all make use of a few simple practical hints.
"We spend a good share of each twenty-four hours in our bedrooms, and it is important to remember that, although we are sleeping, the work of respiration is still going on-it never ceases. It is as essential for us to breathe good fresh air then as when we are awake. Thetop sash of the bedroom window should be kept down an inch or so day and night, for the escape of the foul air. Or a row of holes may be bored in the upper part of the frame with a gimlet, and this will answer equally as well.
"But how about letting in fresh air to take the place of the bad air which escapes in this way? We need not trouble about this. The bad, heated air will be sure to rush away if we leave a means of escape for it in the upper part of the room. The pure air will force itself in through every crack and crevice, through the keyhole, under the door, to take its place as quickly as it passes off.
"As the air which enters the room in this way has to pass through the house, it is important to keep the house and all things round it clean and free from bad smells, or we shall begin by admitting bad air into the room.
"This applies to all workrooms and classrooms, as well as to sleeping-rooms, especially when gas is burned. The only thing to be avoided is the causing of draughts. Draughts create mischief. The best ventilator for a room is a fire; we have seen how the heat of the fire causesinward currents of air to flow from all parts of the room. If a number of persons sit in a room without a fire, the air soon becomes stuffy and foul; but if a fire is burning in the grate, it will help very much to keep the room ventilated.
"A simple way to rapidly purify a close, foul room is to open both the top and bottom sashes of the window at once; the reason for this requires no telling."
