CHAPTER XI 

 Air of Abnormal Composition. 



IN the present chapter I propose to describe the mode of occur- 

 rence and physiological effects of the more commonly occurring 

 gaseous constituents of air. The number of noxious gases, vapors, 

 and particulate impurities, which may, under particular circum- 

 stances, be present in air, is of course very large, and only the 

 commoner additions to air can be dealt with here. 



Outside Air. Pure country air, freed from moisture, contains 

 20.93 per cent by volume of oxygen, .03 per cent of carbon diox- 

 ide, and 79.04 per cent of a residue usually designated as "nitro- 

 gen," although of this 79.04 per cent about .94 per cent consists 

 of argon. Very minute traces are also present of hydrogen and 

 various rare gases. Ordinary atmospheric air contains, however, 

 aqueous vapor in varying proportions ; and about I per cent is on 

 an average present in a climate such as that of Great Britain. The 

 composition of dry country air is the same to the second decimal 

 point all over the world. In summer weather the percentage of 

 CO 2 near the ground may be as low as .025 during the day, and 

 as high as .035 during the night, owing to the influence of vegeta- 

 tion, etc. ; and doubtless the oxygen percentage rises or falls 

 correspondingly, though this has not yet been shown directly. 



In towns the composition of the outside air varies surprisingly 

 little from that in the country. The percentage of CO 2 seldom 

 rises above .05, nor does that of oxygen fall below 20.9, even in a 

 large town, like London ; and in summer weather there is hardly 

 any difference between the oxygen and CO 2 percentages of town 

 and country air. In a London park on a summer day the per- 

 centage of CO 2 may fall quite as low as in the country. Consider- 

 ing the great area of a town like London, and the enormous 

 quantity of coal and gas burnt, this fact is very striking, and 

 shows clearly that apart from horizontally-flowing wind there 

 are very active up-and-down movements of the air, and these 

 keep the air of a town pure. It is only in foggy weather that these 

 up-and-down movements cease more or less; and then the im- 

 purities in the air of a large and smoky town may become very 



