Probably the most important element in air is the oxygen which represents about one-fifth of its composition. This percentage is maintained with very slight variation; for instance, the percentage of oxygen in towns will be slightly less than 20.93%, about 20.87%. When the atmosphere contains only 11% or 12% of oxygen it becomes dangerous, and death results when the percentage gets as low as 7.2%. A candle will not burn with the oxygen at 16%. Slight variation in the amount of oxygen is of no special importance. The amount of oxygen in the air has little to do with the amount absorbed in respiration, as this is governed more by the need of the body than by the amount taken into the lungs. Nature has made a wonderful provision in the protection of the body from extreme changes in the amount of oxygen in the air. This provision increases the degree of intellectual adaptation which is possible in the body. This is shown by the fact that there is a larger amount of oxygen contained at all times in the lungs than is required to supply the red blood cells. It is maintained that the alveolar air contains normally 16% of oxygen. The red blood cells are practically saturated with oxygen as they leave the lungs; however, this amount of oxygen may not be needed by the tissue cells which the red blood cells supply and the amount they absorb upon their return will depend upon what they have given off to the tissue cells. It can readily be seen that the air in the lungs at no time contains the full percentage of oxygen, since one at no time completely exhales the entire amount of air. In this way the residual air loses some of its oxygen and collects carbon dioxide. Animal life is sustained by the oxygen in the air while the carbon dioxide is essential to plant life. The oxygen is carried into the lungs during the inhalation which is produced by the expression of Innate Intelligence through the organs of respiration. The oxygen passes into the blood and is combined loosely with the hemoglobin of the red blood corpuscles; then under the direction of Innate it is carried to all the tissue cells of the body. Here the oxygen leaves the blood and is used in the oxidation which is necessary in the metabolism of the body. The amount of required oxygen varies with different conditions that obtain in the body and is dependent upon age, the activity of the individual and his condition of health. Some authors assert that the average person will inhale about thirty-four pounds of air in twenty-four hours. This would mean a little over seven pounds of oxygen. Only about one-fourth of the oxygen inhaled is absorbed; therefore, according to these figures the individual would absorb on an average of about two pounds of oxygen in twenty-four hours. Nitrogen (N)