It is not improbable, however, that the more rapid evaporation of moisture towards evening may carry with it the volatile particles of corrupted animal and vegetable matter to an extent slightly in excess of that which occurs in the morning, but it is believed these would not equal the greater contamination from burning coals, and the usually greater stillness of the air, producing partial stagnation, so that the air would be a little nearer pure at night than in the day-time. And how unmistakably do all these investigations prove what we ought to have known and accepted without a moment's hesitation, that the Creator, who has made such vast and such minute provisions for supplying every living creature with a constant and copious supply of fresh air, and has made it so important for their existence that they cannot live a moment without it, has made the air at night just as pure and wholesome as in the day-time. We have thus traced the scourge of foul air to our houses, and much of it to our bed-rooms. The next question is, how to get clear of it. We want to know, however, what poisons the air, so as to know in what part of the room it is to be found. We will try a very simple experiment, to show you what a deadly poison the breath is,--to the flame of a candle, at any rate. Here is a simple glass tube, open at both ends--an ordinary lamp chimney--a candle burns freely as you see, and would burn so all night, if it did not burn out. I will now remove the candle, and breathe into the tube through this pipe, and now you see how suddenly the candle is extinguished as I drop it in again. [Illustration: Fig. 3.] Animals are killed suddenly or after a more prolonged struggle, by the exhaled breath, according to the activity or sluggishness with which the blood circulates--a bird would be killed very soon--some partially torpid animals would live a long time. Man has great endurance--struggles long and hard; but if closely confined, will be poisoned to death in one night, as in the case of those confined in the celebrated Black Hole of Calcutta, and on board of vessels where they have been confined below decks in time of a storm. Others will struggle on longer, as in the case of the two thousand and twenty-six who died of consumption last year, in Philadelphia. And now let us see in which part of the room this deadly poison of our breath is mostly found. It is the popular idea, that because the body, and consequently the breath, is warmer than the ordinary temperature of a room, it rises and accumulates at the ceiling. Upon this theory most of our buildings have been ventilated whenever any attention whatever has been given to the subject; but that theory is incorrect; consequently, all practice based thereon is also wrong.