230 COMPOSITION AND PROPERTIES OF ATMOSPHERIC AIR. 



gravity only T o<jo7 that of hydrogen, but a power of heat-conduction 100 times 

 as great as that element, and a density of only T^THF "that of the air). These 

 elements have not been investigated physiologically. ^therium is probably 

 a composite substance, and perhaps plays the role that has been previously 

 ascribed to luminiferous ether. 



Aqueous vapor is always present; its amount usually increases 

 with the height of the temperature. With reference to the humidity 

 of the air there must be distinguished : (a) the absolute humidity, that 

 is, the quantity of aqueous vapor contained in a volume of air; (b) the 

 relative humidity, that is, the quantity of aqueous vapor contained in 

 a volume of air in relation to its temperature. The latter increases with 

 rising temperature. 



The relative humidity is determined either by means of the hygrometer of 

 Klinkerfues or by the psychrometer of August. The latter consists of two accu- 

 rately graduated thermometers, the bulb of one being kept constantly moistened 

 by means of a wet cloth. As a result of evaporation of the water on the bulb, 

 cooling will take place, and the fall of the thermometer will vary directly with 

 the rapidity of evaporation, that is, with the dryness of the atmosphere. From the 

 difference in the readings of the two thermometers the tension of the aqueous vapor 

 in the air may be calculated according to the formula : e = e 1 -k X (t -t 1 ) X b ; 

 in which e represents the desired tension of the aqueous vapor in the air at the pre- 

 vailing temperature, as indicated by the dry thermometer; e 1 the tension of the 

 aqueous vapor that prevails when the air is completely saturated with watery 

 vapor at the temperature of the moist thermometer (to be ascertained from works 

 on physics); b the state of the barometer in millimeters of mercury; t the tem- 

 perature of the dry thermometer, and t 1 that of the moist thermometer (expressed 

 in degrees Centigrade) ; and k an empirically obtained constant = o.ooi. 



Experience has taught that man breathes best in an air that is not completely 

 saturated with watery vapor in accordance with its temperature, but only to 

 70 per cent, of that amount. Air that is too dry irritates the mucous membranes 

 of the respiratory organs; while air that is too moist arouses a feeling of uncom- 

 fortable oppression, and in warmer air a sensation of oppressive sultriness. At a 

 lower temperature (15 C.) dry air is more comfortable than moist air; at from 

 24 to 29 C. dry air feels cooler than moist air. With marked dryness of the 

 atmosphere a temperature of 29 C. is well borne; but exceedingly damp air becomes 

 unendurable for any length of time at 24 C. In the living room and in the sick- 

 room attention should, therefore, be paid to the correct degree of atmospheric 

 humidity. (Sprinkling with water, or in winter placing a basin of water on the stove 

 may be resorted to.) Rooms that are too damp, on account of dampness of the 

 walls or the floor, are prejudicial to health. 



The following factors are known to influence the absolute quantity of aqueous 

 vapor in the air: (i) At the sea-shore during the day the amount is increased 

 with a rising temperature, and diminished with a falling temperature. (2) In the 

 flat, inland country the humidity rises from sunrise to noon, then diminishes until 

 evening; rises again during the first part of the night, and finally falls again. 

 (3) On high mountains the mid-day decrease in humidity does not occur. (4) _ South- 

 western winds in summer are accompanied by the greatest humidity, while east 

 winds in winter bring the lowest degree of humidity. 



With reference to the relative amount of moisture it is to be noted: (i) that 

 it is usually greatest at sunrise, and least toward noon; (2) that it is diminished on 

 high mountains; (3) that it is greater in winter than in summer; (4) that it is 

 usually greater with south and west winds than with north and east winds. 



In the course of the year's changes, that air which is found to be the richest 

 in water absolutely is the poorest relatively. For example, the air in summer 

 contains absolutely about three times as much watery vapor as in midwinter, and 

 still the summer air is relatively dryer than that of winter. In the course of the 

 seasons the absolute humidity rises and falls with the mean temperature. The 

 average relative humidity amounts to about 70 per cent, in temperate climates. 



With increasing elevation above sea-level the density of the air 

 diminishes. 



It likewise diminishes with increase of temperature. 



