LIGHT AND VEGETATION 21 



light. The energy which is not transmitted is partly reflected, 

 often in a diffused way; the rest is absorbed and transformed, 

 especially into heat, after a more or less deep penetration. 



Pure, dry air, up to a thickness of some metres, possesses 

 an almost perfect transparency for all the wave-lengths that 

 we are considering. It begins to absorb again, strongly, only 

 the photons of a wave-length of less than 1,850 A, but these 

 are situated in the extreme ultra-violet which does not appear 

 in the solar radiation that reaches us. (This extreme ultra- 

 violet is produced, however, by sources of artificial light such 

 as the spark, or the mercury arc in a quartz envelope.) The 

 absorption is due to molecules of oxygen. In the infra-red, 

 a partial absorption takes place, due to molecules of carbon 

 dioxide and especially of water vapour. The whole of the 

 atmosphere that the solar radiation has to traverse presents 

 the same absorption bands in the infra-red, particularly in 

 the neighbourhood of the wave-lengths 14,000 A and 19,000 A. 



In addition, we know that all wave-lengths of less than 

 2,900 A are absent from the solar spectrum and that these 

 radiations are absorbed in the upper atmosphere by molecules 



of ozone. 



Ozone is a gas the molecules of which consist of three 

 atoms of oxygen, while the molecule of oxygen has only two. 

 It is produced when ultra-violet photons are absorbed by 

 oxygen (we have just seen that those of a wave-length of 

 1,850 A or below are very strongly absorbed); its odour is 

 perceptible in the neighbourhood of mercury vapour lamps 

 in a quartz envelope. 



A small quantity of ozone, created by the ultra-violet of 

 sunlight, thus exists in the atmosphere. Measured at atmo- 

 spheric pressure, at a temperature of 0° C, this ozone would 

 form a layer of gas 2 to 3 mm. thick, surrounding our globe 

 (the entire atmosphere under these conditions would be 

 10 km. thick). It is this infinitesimal proportion of ozone 

 which cuts the solar spectrum on the ultra-violet side and 

 suppresses all wave-lengths of less than 2,900 A. Later we 

 shall see the extremely harmful effect that ultra-violet rays, 



