It is not only in the schoolroom that you need oxygen. When you are out-of-doors you get an abundance of fresh air, but from a great many houses every bit of fresh air is shut out. It is always possible to let an abundance of fresh air into any house without causing a draft. A piece of board can be made to fit into a window frame so that when the window is raised, the air will be directed upward and will not cause a draft. Hot-air furnaces are made with cold-air pipes. The fresh air from outdoors comes through these cold-air pipes and, after being heated, is driven into the rooms of the house. Some people think they will save coal by closing these drafts. Not only do they not save coal (for the furnace does not give as much heat when this draft is closed), but they kill their body cells by refusing to give them oxygen. The cold-air pipe in a hot-air furnace should always be kept wide open. In houses heated with steam or hot water, either the windows must be kept open, or some other way must be provided for admitting fresh air and taking out foul air. These arrangements constitute a system of ventilation. Houses heated with stoves must also be provided with some means of ventilation. The stove, by its draft, takes out a little of the foul air, but it will not take out more air than one person poisons. [Illustration: FIG. 26. Restfulness: Effect of good ventilation in a sleeping-room, with the right position for sleeping.] [Illustration: FIG. 27. Restlessness: Effect of poor ventilation in a sleeping-room, with the wrong position for sleeping.] [Sidenote: Why windows should be kept open at night] Many people seem to think that they do not need fresh air at night, and they close their bedroom windows as tight as they can. Those people do not sleep well and often have bad colds. You should always sleep with your windows open. If it is impossible for you to have your windows open without having a draft, choose the draft; it will do you no harm if you are well covered, and under no circumstances will it do you as much harm as the foul air that you breathe if your window is closed. Some persons (fortunately they are few nowadays) will tell you that night air is dangerous. I wonder what kind of air these people expect to breathe at night. Do they expect to fill the room in the daytime with enough air for use at night? Such air would certainly not be very fresh. Night air is the only kind of air that it is possible to breathe at night. [Sidenote: The ventilation of public assembly rooms]