AIR AND VENTILATION 



77 



ic combustion of illuminating and heating gases and the various 

 trade and manufacturing processes all take a part in rendering the 

 lir less pure. Factories discharge various gases and substances in 

 suspension into the atmosphere, some of these may be deleterious 

 id some harmless. Chemical works add sulphurous acid, sulphur- 

 etted hydrogen and other gases, but the discharge of such gases 

 ito the atmosphere is regulated by the Alkali Acts, which require 

 lat, in the case of hydrochloric acid gas, not more than one-fifth of 

 grain per cubic foot of air, smoke, or chimney gas may pass out 

 ito the atmosphere. Of acid gases of sulphur and nitrogen, not 

 lore than the equivalent of four grains of sulphuric anhydride 

 ;r cubic foot of air must be discharged into the atmosphere. Free 

 imonia is found in the air of badly constructed and badly ven- 

 tilated animal houses, and results from the decomposition of urea. 

 Jnder good hygienic conditions and with good stable management 

 it should not be present in appreciable amount. The evaporation of 

 foul water from cess-pits and stagnant pools may add gaseous 

 products to the air. 



At each respiratory act the inspired air loses about 4| per cent, 

 of its oxygen and gains 4 per cent, of carbon dioxide. The nitro- 

 gen and other gases remain substantially the same. Thus the com- 

 >sition of expired air is : Oxygen, 16 4, carbon dioxide, 4 1, 

 nitrogen, 78 09, and argon, &c., 94 per cent. Of great import- 

 ance is the physical change, as after inspiration the air is raised to 

 the temperature of the animal body and is saturated with moisture. 

 This fact is clearly seen on cold days when condensation takes place 

 as the warm, moist exhalations come in contact with the colder and 

 drier atmosphere. As air becomes warmed expansion takes place, 

 therefore the air of expiration has a greater volume than that of 

 inspiration. This fact is the basis of all systems of natural 

 ventilation. 



OXYGEN DECREASE. The nitrogen and other inert gases may be 

 neglected so far as a study of air pollution is necessary, since they 

 play no part in rendering the air harmful to either mankind or 

 animals. There is reason to believe that oxygen, though it is 

 decreased over 4 per cent, at each respiration and retained in the 

 tissues, may also be regarded as a negligible factor among the many 

 which either singly or by their combined effects may be deleterious. 

 It is known that the amount of oxygen may be reduced to less than 

 17 per cent, before its diminution becomes harmful to man. Air 

 containing so low a proportion is unable to support combustion. In 

 greatly over-crowded rooms it is said rarely to fall below 20 per cent. 

 While it is now safe to assume that the actual diminution in the 



