THE RESPIRATORY EXCHANGE 481 



The partial pressure, or, as it is often called, the tension, of 

 the component gases is : 



21 

 Oxygen approximately y^ x 760= 159'6 mm. of mercury or 



21 per cent, of an atmosphere. 



79 

 Nitrogen approximately y^ x 760 = 600'4 mm. of mercury or 



79 per cent, of an atmosphere. 



Carbon dioxide approximately y x 760 = 0-228 mm. of 

 mercury or 0'03 per cent, of an atmosphere. 



Such is the composition of pure country air. Many men, 

 however, are obliged to breathe air which is vitiated by the pro- 

 ducts of their own respiration, the combustion of gas or coal, dust 

 produced during work, or chemical changes occurring in the soil. 

 A knowledge, therefore, of the composition of the air of dwelling- 

 rooms, workshops, schools, mines, and crowded cities is of the 

 greatest practical importance. 



Air of Dwellings, &c. The impurities present in a workroom 

 arise from the workers, lights or fires, the floor and furniture of 

 the room, the raw material and accidental sources, such as escapes 

 of gas. The carbon dioxide is in practice the best indicator of 

 the sufficiency of ventilation ; but it will be shown later that it 

 is only one of the impurities present in overcrowded rooms. The 

 proportion of carbon dioxide will vary according to the sources 

 of contamination and the ventilation of the room ; it may be 

 during the daytime as low as 3-2 volumes, or as high as 33*6 

 volumes per 10,000 volumes of air, and at night-time it may rise 

 as high as 47 volumes, if many gas-lights be burning. Haldane 

 and Osborn ( 3 ) have investigated the ventilation of a large number 

 of workshops and factories, and recommend that the standard of 

 ventilation should be such that the proportion of carbon dioxide 

 does not exceed 12 volumes per 10,000 of air during daylight, and 

 20 volumes after dark, when gas or oil is used for lighting. During 

 a dense and prolonged fog this proportion may be exceeded even 

 in the open air of a large city, for Russell found as much as 10 to 

 14 volumes of carbon dioxide per 10,000 volumes of air in samples 

 collected in London during very foggy weather. 



Expired Air. In the healthy man or animal the air is inspired 

 through the nose, and only during severe exercise is prolonged 

 buccal breathing to be considered normal ; buccal respiration 



2 H 



