22 



LIGHT, VEGETATION AND CHLOROPHYLL 



absorbable by ozone, have on vegetation. We know also that 

 they scorch the human skin and that their powerful action is 

 used, with caution, in medicine for the treatment of certain 

 diseases. 



These 2 or 3 mm. of ozone, by their presence and the 

 thin screen of their molecules, therefore protect animal and 

 vegetable life from the action of these dangerous ultra-violet 

 rays of short wave-length, but they permit the passage of 

 small quantities of rays, of wave-lengths a Uttle greater than 

 2,900 A, which still have a slight lethal action on bacteria and 

 fungi. They cause "sunburn", but they purify our globe and. 



Fig. I, 3. Transmission curves for pure water and for a 

 solution of cupric chloride in water for the visible and near 



infra-red radiations 



without them, we should perhaps be submerged by decaying 

 matter. This shows the admirable equilibrium of adaptation 

 between life and the conditions of radiation in which it 

 develops. 



A shallow depth of water is transparent enough to visible 

 and ultra-violet light, but it is opaque to infra-red of a wave- 

 length greater than 13,000 A (or 1-3 /^t), i.e., to nearly all the 

 infra-red in which we are interested (Fig. I, 3). 



Particles dissolved or suspended in natural water (lakes, 

 rivers and seas) greatly reduce its transparency, especially to 

 ultra-violet. Thus it has been found that in a lake of clear 

 water there is about 100,000 times less visible Ught at a depth 

 of 30 m. than at the surface. But an ultra-violet lamp can 

 act on algae which are submerged at only a shallow depth. 



