One interesting result of recent inquiries on this subject is the discovery that the bad effects of crowded, “stuffy” rooms are not generally due to impurities in the air, but to heat, humidity, and especially lack of air movement. It seems to be now demonstrated that there is no such thing as “crowd-poisoning,” and that the bad smells of confined places are no indication that the air is deleterious. Professor Hill, who has done more than anybody else to upset traditional ideas with regard to ventilation, tells us that “the deaths in the Black Hole of Calcutta, the depression, headache, etc., experienced in close rooms are alike due to heat stagnation; the victims of the Black Hole died of heat stroke.” Most recent writers on physiology also discredit the time-honored methods of testing the purity of the air by measuring the percentage of carbon dioxide it contains. The amount of this gas normally present in the free air is about three-hundredths of one per cent, but experiments have shown that thirty times this amount--a percentage higher than is found in the worst ventilated rooms--may be breathed for hours together without detrimental effects. A further departure from old-fashioned views is seen in the assertion of recent authorities that a deficiency of oxygen, unless far more pronounced than ever actually occurs in buildings, mines, etc., where the supply of this gas has been the subject of so much solicitude, has no physiological significance whatever. In support of this assertion it is pointed out that at mountain health resorts the concentration of oxygen out-of-doors is much less than that found in the worst ventilated rooms at sea-level. In mines an ample supply of oxygen may be positively dangerous, as favoring the occurrence of explosions. These were rare before the enactment of laws requiring a high percentage of oxygen in mine air. [Illustration: HILL’S KATATHERMOMETER]