The amount of carbonic acid in the breath is about five per cent. Air once used is therefore unfit for purposes of animal combustion. If breathed into a jar containing a short lighted candle, it will at once extinguish the flame. It would also prove fatal to small birds or mice. When the carbonic acid reaches one part in ten of common air, it becomes fatal to man. Headaches, dullness, drowsiness, and labored respiration are the first symptoms of this lung poison. Faintness, convulsions, and unconsciousness are a later stage. School-houses, churches, theatres, and factories should be so well ventilated that the proportion of carbonic acid would not exceed two parts in one thousand. =Effects of Breathing Impure Air.=--Air which is only slightly vitiated, if breathed day after day, for a considerable time, produces most serious results. Its effects are seen in pale faces, loss of appetite, depressed spirits, and a lack of muscular vigor. An investigation made some years ago showed 86 deaths per 1,000 in a badly ventilated prison, and of these, 51.4 per 1,000 were due to phthisis, or consumption. In the House of Correction, in the same city, which was well ventilated, the death rate was 14 per 1,000, and of these only 7.9 were occasioned by phthisis. The organic particles thrown off from the lungs of diseased persons are responsible for the prevalence of phthisis and other lung diseases. It is also a well established fact that a bad atmosphere promotes the rapid spread of such specific diseases as small-pox, typhus, and scarlet fever. =Constant Supply.=--Of so great importance is the matter of having a constant supply of unvitiated air that sanitariums for consumptives are now becoming common in which the principal feature is to have the patients enjoy a continuous out-door existence, day and night, being wrapped up and otherwise protected from cold and dampness. Consumptive symptoms often yield to this treatment. =Individual Habit.=--Habit has much to do with our appreciation of pure air. If we recognize its value to health and to all the mental and physical activities, and insist upon a plentiful supply of pure oxygen, the habit soon becomes a second nature, and we instinctively feel uncomfortable upon entering an ill-ventilated room. In northern climates, economic considerations often interfere with the highest sanitary regulations. Householders, school boards, and church trustees frequently save fuel at the expense of health.