=Necessity for a Constant Supply of Pure Air.= When we consider that the food we eat and digest cannot nourish the body until it has been acted upon by oxygen in the lungs, and that this action must be constant, never ceasing, it will help us to understand the necessity for a constant supply of air such as shall furnish us a due proportion of the life-giving principle, oxygen, and which shall not contain impurities that interfere with its absorption. We take into the lungs a mixture of nitrogen, oxygen, and carbonic acid. We give out a mixture which has lost some of its oxygen, and gained in carbonic acid. Now, unless the amount of oxygen is what it should be, the blood will not gain from an inspiration the amount it should receive, consequently it will be but imperfectly purified and able but imperfectly to nourish the body. So the whole system suffers, and if a person for a long time continues to breathe such an atmosphere, the condition of the body will become so reduced as to produce disease. Even though in other ways one lives wisely, all the factors of health multiplied together cannot withstand the one of impure air. We eat food three or four times daily. Some of us are very particular about its quality. We breathe air every instant of our lives, but generally we give but little consideration as to whether it is pure or impure. =Ventilation.= No attempt will be made here to explain different devices for ventilation, but only to touch upon the principle it involves. Its objects are (1) to remove air which has been breathed once; (2) to remove the products of combustion, whether from fires, lamps, gas, or other sources; (3) to carry away all other substances which may be generated from any cause, in a room or building, as the impurities from manufacturing, those arising from decaying matter, and micro-organisms. In a climate where artificial warmth is necessary a part of the year, it is difficult to warm and ventilate a room at the same time, without causing unpleasant drafts; but with some knowledge of the necessity of ventilation, and of the properties of air, one may in some measure work out a scheme of ventilation adapted to the circumstances in which he finds himself.