Second, a wood or coal or gas stove in the room, provided with a proper vent, so that all the gases of combustion are drawn up the chimney. This changes the air more slowly than an open fireplace, but it does the work fairly well. All that you have to be careful about is that your vent is sufficiently large and is working properly. If your fire does not "draw," you will have smoke or coal-gas in the house, and this is bad for the lungs; but worse for the lungs is a gas that you can neither see nor smell nor taste, the deadly carbon monoxide. This gas is produced by incomplete combustion, and whenever you see yellow flames from gas or coal, you are apt to have this poisonous substance. Small quantities of it are sufficient to cause violent headaches, and repeated doses of it are fatal. Men who work in garages which are not properly ventilated run this risk all the time, because carbon monoxide is one of the products of imperfect combustion in the gas engine. Next, the furnace. A furnace sends fresh warm air into your house; the only trouble is that it takes out all the moisture, and some authorities say that this is bad for the lungs and throat. I do not know whether this is true, but all furnaces are supposed to have a water chamber to supply moisture to the air, and you should keep a pan of water on every stove or radiator in your house. Next, steam heat, which includes hot-water heating. This is one of the abominations of our civilization, and one of the methods by which our race is committing suicide. There is nothing wrong about steam heat in itself; the room is warmed in a harmless way; but the trouble is it stays warm only so long as the doors and windows are kept shut. You are in an air-tight box, and can be warm provided you do not mind being suffocated. The moment you open a door or window, you have a cold draft on your feet, and if you wish to change the air entirely you have to let out all the heat; so, of course, you never do change it entirely, but go on breathing the same air over and over, and every time you breathe it the condition of your body is a little more reduced.