As soon as some arrangement of this kind has been completed, you will find that the air begins to move gently through the room, cold fresh air coming in through the holes in the door, and warm impure air being pressed out before it through the perforated zinc tubes or plates. This takes place partly because the external wind rushes, in its hasty way, against the openings through which the air is intended to enter, and forces itself in; but also, and more particularly, because the inside air gets warmer than the outside, and is then compelled to shift its quarters on that account. The air contained inside of inhabited rooms gets warmed by the bodies and breaths of the persons living there. Then it is lighter, bulk for bulk, than the colder air outside, for warmth stretches and lightens everything. But as heavy things fall or press down to the earth more strongly than light ones, the cold air always squeezes into the room through the lower openings, and pushes the warm impure air out before it, through the upper ones. When you light a fire in your room during cold weather, it makes a quick and strong draught through the room, for the same reason. Fires, indeed, are among the most powerful ventilators that can be brought into play. Let your fire out, and go on sitting in the room with two or three of your neighbors, and you will find the air of the room will be close and foul in half-an-hour, although it was quite fresh before. While the fire is burning, the chimney takes upon itself the office of the holes in the zinc tube or zinc plate fixed in the window, and the heated air of the room is pushed up through it by the fresh cold air which rushes in through all other openings and crevices. It is only in rooms where no fires are burning—as for instance, in your sleeping room—that holes through the walls and windows can serve as outlets for impure air. But if you live with several companions, in small rooms, as some workpeople are compelled to do by their occupation, those rooms cannot get properly ventilated, even although fires are burning. Some of the poison-vapors, poured out from your living bodies with the breath, are so light that they are at once driven up to the top of the room, and collect there gradually, spreading lower and lower as they become more abundant. They can not get out through holes made in the walls or windows because, as we have seen, the fire causes streams of cold air to press in there. A plan has been contrived, however, to ensure perfect ventilation even in small and crowded rooms, provided fires be burning. This plan consists in making an opening into the inside of the chimney, near to the ceiling, and fixing a balanced valve in it in such a way that the valve-plate is opened by outward draughts, but immediately closed by inward ones. Then the impure vapors lurking near the ceiling are continually being swept away, into the current of the chimney, through this valve.