If we now take such tuberculous persons and place them in a hospital or sanatorium and subject them to the open air treatment, compelling them to pass all their time exposed to the fresh air, both day and night, we soon witness a marvelous change. Their appearance is improved, and also their appetite; and after a time in most of the cases there will be an increase in bodily weight. We thus see that the open air produces wonderful effects in such persons, who, as a rule, have been immured in close places, they shattering their health. We have thus witnessed the clinical demonstration of the fact that fresh air is able to improve our health. Fresh air contains much oxygen, and this is a most indispensable substance, for without it we cannot live. The red corpuscles in the blood which, loaded with carbonic acid, the veins convey to the lungs, eagerly absorb the oxygen from the air that we inhale and then convey it to the tissues to satisfy their requirements for this precious substance. By absorbing oxygen the elimination of carbonic acid is at the same time facilitated. The greater the number of red blood corpuscles that comes to the surface of the lungs the greater will be the volume of oxygen which is taken into the system, and afterwards the larger will be the volume of carbonic acid gas expelled. Thus in the lungs there takes place a distintoxication of the organisms, and, according to certain authorities, the cells of the lungs are co-operating in this process in a manner analogous to the internal secretion by the cells of other glandular structures. The more fresh air, i.e., the more oxygen we get into our lungs, the more we can contribute to the processes of oxidation in the tissues. When the processes of circulation and of breathing are checked, and when insufficient oxygen is absorbed, we soon see a very important change for the worse in the condition of such persons, as exemplified by cases of heart and lung trouble. Given the great importance of oxygen, we must try by every means to get as much of it into our lungs as possible; we shall get more of it from air that is not stagnating, but always in circulation. When we are in a closed room, after a certain time we absorb all the oxygen in it, particularly when there are several persons present who are sharing with us the oxygen in the air. Staying for a long time in air so vitiated that it contains but little oxygen and much carbonic acid and many microbes exhaled by the others, we are liable to reap the disadvantages we have set forth in the chapter on the dangers of a close room. According to Pettenkofer, the exhalations from the persons present in a close room are much more noxious than the carbonic acid gas. We, therefore, open the window and door in order to create a current of fresh air, and so allow the oxygen to be renewed. In this we but imitate nature, which sends a wind to purify the close atmosphere on warm summer days. This is natural ventilation.