THE INFLUENCE OF URBAN AND RURAL ENVIRONMENT 359 



have created in our cities. We are largely in control of 

 environment but we do not yet know w^ith certainty the 

 lengths to which we can safely go in modifying it for our 

 convenience, comfort or pleasure. 



THE ATMOSPHERE 



The atmosphere is man's most intimate physical environ- 

 ment. Physiologically it has two primary functions: It 

 provides for the necessary respiratory exchange, oxygen 

 being taken into the system, and carbon dioxide being given 

 out. It also provides for the removal of heat from the body 

 surfaces, lungs and skin, by processes of convection and 

 evaporation. Except under conditions of asphyxiation, 

 smothering or drowning, the respiratory exchange function 

 seldom fails. The modern problems of ventilation concern 

 themselves to a large extent with the second or heat-remov- 

 ing properties of the air. 



As has been previously suggested, the air provides a 

 chmate, and under our present-day habits of hfe, especially 

 in the cities, we deal largely with an artificial chmate. 

 In addition to the temperature, humidity and movement of 

 the air, its three significant physical properties affecting 

 comfort and health, the atmosphere has quahties that 

 determine the character and extent of solar radiation reach- 

 ing the earth's surface. It is in fact a selective screen through 

 which the sun's rays pass with more or less modification. 

 Of the hght of the visible spectrum, "hght" in the common 

 use of the word, about 20 per cent is absorbed by a clear 

 atmosphere at sea level. The rate of absorption increases 

 with decreasing wave length so that only a small part of the 

 total ultraviolet radiation of the sun ever reaches the earth's 

 surface, while a large proportion of the infra-red spectrum 

 and the heat rays do come through. This selective screening 

 effect is of course modified by the thickness of the air 

 layer (altitude of the place) and by the clouds, fog, smoke and 

 dust. 



Another property of the atmosphere, concerning the 

 effect of which we know but httle, is its electrical property. 

 We hve in a strong potential gradient by reason of which 

 an electric current is always passing between the earth 



