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Various experimental researches, and especially the latest and very careful investigation by Haldane, Douglas, Henderson and Schneider on Pike's Peak, have proved beyond a doubt that that formerly mysterious disease, mountain sickness, is due solely to the greatly diminished pressure of oxygen existing at all considerable heights. That wonderful power of adaptation to unusual conditions, however, of which the human body is so generously possessed, is here demonstrated in the fact that on reaching the unusual height the quantity of hemoglobin, which gives the red color to the blood and enables it to carry oxygen to our tissues, begins to increase, and a few days of life at the high altitude renders us capable of continuing to live there under the diminished oxygen pressure without further danger to life.

Within a crowded assembly the proportion of oxygen may fall to one twentieth of its usual amount in the outdoor air, probably never more except in the most extreme experimental conditions. Experimentation has apparently shown that the evil effects of such indoor air are not due in any respect to this slightly lesenedlessened [sic] quantity of the gas. It has even been diminished to less than seventeen per cent, in experimental chambers without apparent detriment to persons confined therein. Hill says of a group of his students whom he confined in a narrow air-tight room: "We have watched them trying to light a cigarette (to relieve the monotony of the experiment) and puzzled by their matches going out, borrowing others, only in vain. They had not sensed the percentage of the diminution of oxygen, which fell below seventeen." The ventilation of coal mines by air containing only seventeen per cent, of oxygen has indeed been suggested as a preventive of explosions. On the other hand, a "sand hog" working in a caisson at a depth of one hundred feet must be subjected to a pressure of oxygen four times that found in the usual atmosphere. Here he can work for several hours with impunity; a longer time would give an opportunity for the excess of this gas to manifest its toxic action on his tissues. Because of this poisonous action too, a man can breathe pure oxygen when in excess for a limited period only. The administration of oxygen in extreme illnesses thus has its limitations.