PLANKTOiN". 285 



have passed through the water. But in absohitely pure 

 water the rays would not be reflected, but would pass 

 straight on. Therefore water must contain a certain 

 amount of impurities to appear coloured to us. Suspended 

 particles are necessar^' to reflect the light. Tlie smaller 

 and fewer these particles are, the deeper does the light 

 penetrate the water before it is reflected, and the bluer 

 it appears when it reaches our e3'e. So that the geo- 

 grapher Kriimmel is justified in saying that the more 

 clear and transparent the water, tlie purer its blue. The 

 nortlaern seas are as a rule less clear than the southern 

 ones, because in warmer and Salter water the particles 

 of foreign matter sink to the bottom c|uicker: thus it is 

 that the southern seas are famoLiS lor their blue colour. 

 As lakes are more liable to be charged with impurities, 

 they rarely fulfil the conditions necessary for blue hues. 

 It has lately been discovered that plaiihtoii, that swarm 

 of minute organisms which live suspended in the water, 

 contributes largely to the impurit-^- of Jakes and especially 

 of the sea. The northern seas are the richest in plankton. 

 The eastern end of the Mediterranean has been found 

 to be particularly poor in plankton, and this is just where 

 the blue is the most intense. Some physicists have 

 suggested that the fineness of the particles suspended in 

 the water may also contribute to the depth of its blue 

 colouring. For the smaller the particles the more imper- 

 fectly are the long-waved red rays, and the more per- 

 fectly the short-waved blue rays reflected : and thus the 

 blue colour is intensified. This same phenomenon, taking 

 place in our atmosphere, is said to be the cause of the 



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