Let us note now, briefly, some points of primary importance in the conditions that assure general health. Air, warmth and light must be provided for the dwelling. In cities we cannot always choose, but in smaller communities and in the country we can in large degree control such things for ourselves. Some things require only to be suggested to be clearly understood. A house should stand where the character of the soil and the contour of the surface will provide the best drainage. Hollows should be avoided. When a house is built on a hillside the ground should not be dug out so that a cliff rises immediately behind. Trees may afford valuable shelter, not only from cold winds, but from fogs. But it is not generally wise to have them close around a dwelling, at least in large numbers, since they impede the free circulation of the surrounding air, and retain dampness beneath their shade. In the country a house may be sheltered from cold winds on the side from which they prevail, by trees. Exposure of each side of a house in succession to the rays of the sun helps to keep the outer walls dry, to warm it in winter and to aid ventilation in the summer. The north wall may be made with advantage a dead wall, and ventilating pipes and soil pipes may be carried up through it, but chimneys carried up through a north wall, being warmed with difficulty and apt to smoke, should not project but be built inside the house. Attics with slanting ceilings and dormer windows are cold in winter and hot in summer. Once occupied, the most important thing in the house is fresh air. The most common impurity in the atmosphere of rooms is carbonic acid gas, which is thrown off by the lungs of the occupants, and must be disposed of by ventilation in order that health shall be assured. The lamps or gas lights used in the room likewise give off carbonic acid, which is formed at the expense of the oxygen of the air, the vital element, which we require to breathe. Crowded rooms, or any rooms improperly ventilated, become tainted in this manner, and the headaches and faintness which we experience under such circumstances are direct and natural results of carbonic acid poisoning. School rooms are particularly trying upon pupils and teachers, unless their ventilation is especially guarded. It is considered that the proper degree of purity in the air of a room can be maintained only by introducing at least 2,500 cubic feet of pure air per hour for each person, this being a virtual minimum. In mines it has been noticed that the men require not less than 6,000 cubic feet per hour, and that when the quantity falls to 4,000 cubic feet there is a serious falling off in the work done. Manifestly the better and tighter the building the more need there is for special means of ventilation.