254 COMPOSITION AND PROPERTIES OF ATMOSPHERIC AIR. 



tube, B, filled with baryta solution, which takes up the C0 2 . The quantity of 

 air which passes through the accessory current, n, is measured by the small gas- 

 meter, u, from which it passes outwards. The second accessory stream, N, enables 

 us to investigate the air before it enters the chamber, and it is arranged in 

 exactly the same way as n. 



The increase of CC>2 and Hg O in the accessory stream, n (I.e., more than in N), 

 indicates the amount of COj given off by the pressure in the chamber, Z. 



123. Composition and Properties of Atmospheric Air. 



1. DRY AIR contains : 



Gas. By Weight. By Volume. 



O, . . 23-015 20-96 



N, 76-985 79-02 



C0 2 , . . . 0-030-034 



2. AQUEOUS VAPOUR is always present in the air, but it varies 

 greatly in amount, and generally increases with the increase of the 

 temperature of the air. In connection with the moisture of the air we 

 distinguish (a), the absolute moisture, i.e., the quantity of watery vapour 

 which a volume of air contains in the form of vapour ; and (fy, the 

 relative moisture, i.e., the amount of watery vapour which a volume of 

 air contains with respect to its temperature. 



Experience shows that people generally can breathe most comfortably in an 

 atmosphere which is not completely saturated with aqueous vapour according to its 

 temperature, but is only saturated to the extent of 70 per cent. If the air be too 

 dry it irritates the respiratory mucous membrane ; if too moist, there is a disagree- 

 able sensation, and if it be too warm a feeling of closeness. Hence, it is important 

 to see that the proper amount of watery vapour is present in the air of our sitting- 

 rooms, bedrooms, and hospital wards. 



The absolute amount of moisture varies : In towns during the day it increases 

 with increase of temperature, and diminishes when the temperature falls; it also 

 varies with the direction of the wind, season of the year, height above sea-level. 



The relative amount of moisture is greatest at sunrise, least at midday ; small 

 on high mountains; greater in winter than in summer; larger with a south or a 

 west wind than with a north or an east wind. 



The air in midsummer contains absolutely three times as much watery vapour 

 as in midwinter, nevertheless the air in summer is relatively drier than the air in 

 winter. 



3. The air EXPANDS BY HEAT. Rudberg found that 1,000 volumes 

 of air, at 0, expanded to 1,365 when heated to 100C. 



4. The DENSITY of the air diminishes with increase of the height 

 above the sea-level. 



124. Composition of Expired Air. 



1. The expired air contains MORE C0 2 in normal respiration = 4'38 

 vols. per cent. (3'3 to 5 -5 per cent.), so that it contains nearly 100 

 times more C0 2 than the atmospheric air. 



2. It contains LESS O (4'782 vols. per cent, less) than the atmos- 

 pheric air, i.e., it contains only 16'033 vols. per cent, of 0. 



