Dr. Stark, Secretary to the Meteorological Art Society of Scotland, states that in Great Britain a certain amount of moisture in the air relative to its temperature is essential to health, and a deficiency in this amount is followed by an increasing mortality. By this is not meant the absolute amount of aqueous vapor in a cubic foot of air, but its relative amount. Thus, at a temperature of 30 Fah. a cubic foot of air requires about two grains in weight of watery vapor to saturate it completely. But if the heat of that air be raised to 60 , it requires rather more than tf grains in weight of aqueous vapor to produce the same amount of saturation. Yet both these airs are in the same relative state as to saturation with moisture, both have just that amount which they can easily carry. Meteorologists have agreed to reckon full saturation of the air with moisture, whatever be the temperature, as 100; and in Scotland the degree of humidity which appears to be most conducive to health ranges from 80 to 86 Thus, air at the temperature of 30 , with one grain and six-tenths of aqueous vapor, would be in the same state, as to moisture, as air at 60 with 4J grains in weight of watery vapor —both would indicate 50 of humidity, and be in the best condition, in so far as amount of moisture is concerned, for the support of health.
This article was originally published with the title "Air and Moisture" in Scientific American 13, 37, 289 (May 1858)