Everything of which the body makes use is taken in the form of food and drink, with the exception of one substance, the oxygen we get out of the air. Every time we draw a breath we take in a certain amount of oxygen, and every time we expel a breath, we drive out a certain amount of a gas called carbon dioxide, which is what the body makes of the fuel it burns. The body can get along for several days without water, and for two or three months without food, but it can only get along for two or three minutes without oxygen. It should be obvious that when the body expels carbon dioxide, with a slight mixture of other more poisonous gases, and sucks back what it expects will be a fresh supply of oxygen, it wants to get oxygen, and not the same gases it has just expelled, nor gases which have been expelled from the lungs of other people. In the days when primitive man lived outdoors, he did not have to think about this problem. When he breathed poison from his lungs, the moving air of nature blew it away, and the infinite vegetation of nature took the carbon dioxide and turned it back into oxygen. And even when man built himself shelters, he was not cunning enough to make them air-tight; he had to leave a big hole for the smoke to get out, and smaller holes through which to get light. But now our wonderful civilization has solved these problems; we make our walls of air-tight plaster, and we have invented a substance which will admit light without admitting air. So we have the "white plague" of tuberculosis, and so we have innumerable minor plagues of coughs and colds and sore throats. In the summer time the solution of the problem is easy. Have as many doors and windows in your home as possible, and keep them open, and have nothing in your home to make dust or to retain dust. But then comes stormy and cold weather, and you have to close your doors and windows, and keep your home at a higher temperature than the air outside. How shall you do this, and at the same time get a continual supply of fresh air? I will take the various methods of heating one by one. The problem in each case is simple and can be made clear in a sentence or two. First, the open fireplace. This is a perfect solution, if you have enough fuel, and do not have to worry about the waste of heat. An open fireplace draws out all the air in the room in a short time, and you do not have to bother about opening doors or windows; you may be sure that the air is getting in through some cracks, or else the fire would not burn.