CHAPTER II. 

 THE EXTERNAL ENVIRONMENT. 



164. The Atmosphere. Human life develops in an atmospheric 

 environment which extends to a distance of 150 to 160 kilometres 

 from the earth's surface. At the inhabited levels of the earth, 

 the composition of the air, not counting rare gases and impurities, 

 is : 



Nitrogen ( x ) : 79% Oxygen : 21% (in volumes). 



There is a trace of carbon -dioxide about 0-3%. The amount 

 of water vapour in the atmosphere varies and gives the hygro- 

 metric state of the environment (see 173). Finally, microbes, 

 solid and invisible particles, " electric grains " ( 2 ) are disseminated 

 in the normal atmosphere. Accidentally the air may contain 

 more or less toxic or deleterious gases (sulphurous acid gas, 

 chlorine, carbon-monoxide, etc.). 



The purity of the air increases with altitude ; the disturbing 

 effect of the nearness of the earth from this point of view, can 

 still be felt at a height of 3 kilometres. Clouds are found below 

 this level and up to 10 kilometres. ( 3 ) 



The temperature of the air varies in the twenty-four hours : 

 the maximum occurring towards 2 o'clock in the afternoon, and 

 the minimum about sunrise, at 8 o'clock in the morning in 

 January, and at 4 o'clock in the morning in July : 



There is, in consequence, a variation of temperature equal to 

 3-6 C. in January and 9-3 C. in July. 



The temperature of the air varies with altitude and latitude. 



The weight of air pressing on the surface of a body (that of a 

 man, for instance) at a given altitude, is called the atmospheric 

 or barometric pressure. The normal pressure at sea level is 

 capable ot sustaining a column of mercury 760 millimetres in 

 height. The weight of a cubic centimetre of mercury being 13-6 

 grammes, the corresponding weight of the barometric column is 

 1033 grammes. The average surface of an adult's body being 

 about 2 square metres, the total atmospheric pressure on the 

 body is approximately : 



20,000 X 1,033 = 20,660 kilogrammes. 



(!) This includes 0'94% of argon, according to Leduc. 



( 2 ) A term due to M. Langevin. They are usually called ions. 



( 3 ) See Wegener's interesting monograph, Fortschritte der Naturvissen- 

 schaft-forschung der Abderhalden, vol. iii., 1911). 



