BLUE WATER 79 



to know more precisely the precautions to be taken in 

 order to get the blue colour in this way in fullest degree. 



It appears that when light is reflected from a cloud of 

 fine colourless particles so as to give a predominant blue 

 colour, the light so reflected is affected in that special way 

 which physicists describe as being "polarized." It is 

 possible by the use of certain apparatus (the polariscope) 

 to distinguish polarized from non-polarized light, so that 

 it should be possible to decide (or at any rate to gain 

 evidence) whether blue water — a sheet of blue water — 

 owes its colour to fine particles suspended in it or to the 

 self-colour of the water. An admirable case for making 

 this simple experiment is presented by the great tanks — 

 some 20 ft. cube — which are used by the water companies 

 which draw their water supply from the chalk, for the 

 purpose of precipitating the dissolved chalk — " Clarking " 

 the water, as it is called, after the inventor of the process 

 — and so getting rid of its excessive "hardness." Such 

 tanks are to be seen by the side of the railway near 

 Caterham. The water in these tanks is of such a 

 brilliant turquoise blue that many people suppose that 

 copper has been added to the water to free it from 

 microbes ! Such, at any rate, was the conviction expressed 

 by a friend in conversation with me only a few weeks ago. 

 The water in these tanks, when seen from the railway, looks 

 like a magnificent blue dye, and a very important point 

 for those (not a few) who believe that the blue colour of 

 seas and lakes is due to the reflection of the blue colour of 

 the sky overhead is that the water in the tanks looks just 

 as blue when the sky is overcast with cloud as when 

 there is blue sky. The blue colour of water has, as a rule, 

 nothing to do with the reflection of the sky, though it is 

 the fact that a shallow film of water may at a certain 

 angle reflect the sky to our eyes, just as a mirror may. 

 The effect is quite unlike that due to light passing through 



