CHAPTER I 



THE BIOSPHERE 



The terrestrial globe is surrounded by an atmosphere consisting chiefly of 

 nitrogen, oxygen and argon, but containing other elements and such com- 

 pounds as carbon dioxide. 



The distribution of molecules in the atmosphere is of considerable bio- 

 chemical interest. At altitudes below 20,000 m the average composition 

 shown in Table I is maintained constant by convection currents, but at 

 higher altitudes, due to the difi^erent molecular weights of the gases compos- 

 ing the atmosphere, a separation by sedimentation becomes apparent. 

 Beyond 150,000 m, this separation becomes very marked and progres- 

 sively the atmosphere becomes less dense until finally it fades into the 

 emptiness of interplanetary space. Nearest to the earth is the troposphere, 

 its composition being kept more or less constant at the values shown in 

 Table I, by convection currents. Its thickness varies from 10 to 15,000 m 

 according to the season and the latitude. Above it is the stratosphere, so- 

 called because of the stratification of different gases in order of their 

 molecular weights. The layers of gas are not disturbed by convection 

 currents and there is no appreciable circulation of molecules in a vertical 

 direction. Whilst in the troposphere the temperature decreases with alti- 

 tude, in the stratosphere it is independent of the latter. 



At around 80,000 m is the start of the ionosphere which takes its name 

 from the fact that it is rendered conducting by the ionization phenomena 

 produced by the sun's ultra-violet radiation. The presence of carbon di- 

 oxide in the troposphere, as we shall see, is very important despite its low 

 concentration. The presence in the stratosphere of a diffuse layer of ozone, 

 the ozonosphere, is not less important for it prevents the greater part of 

 the sun's ultra-violet radiation from reaching the surface of the earth where 

 it would otherwise soon put an end to all life. 



The earth is also surrounded by a hydrosphere, a discontinuous layer of 

 water in different physical states which separates the lithosphere from the 

 atmosphere and extends into the latter in the form of water vapour. 



The hydrosphere is made up of oceans, lakes, rivers, streams, water 

 absorbed by the rocks and by snow and ice. Oceans cover 70% of the 

 earth's surface and their average depth is 3,800 m, and sea water represents 

 approximately 98% of the hydrosphere. The composition of the sea 

 water and its dissolved gases is given in Table II. 



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