80 SECRETS OF EARTH AND SEA 



deep water when reflected from below it. If we examine 

 the tanks in question we find that they have been filled 

 with water pumped from the chalk, and that then lime has 

 been added to the water in order to combine with the car- 

 bonic acid dissolved in it and form chalk or carbonate of 

 lime — which is insoluble in pure water and falls as an 

 excessively fine white powder to the bottom of the tank. 

 But the important fact is that water having carbonic acid 

 dissolved in it can dissolve carbonate of lime or chalk to a 

 certain amount : and this water pumped from the chalk, 

 having carbonic acid naturally dissolved in it, has conse- 

 quently also dissolved a quantity of chalk. It is this 

 which gives the chalk-spring water the objectionable 

 quality of " hardness." When lime is added .to the chalk- 

 spring water as pumped into the tanks, the carbonic acid 

 in it is taken up by the lime, and the chalk previously 

 dissolved by the carbonic-acid-holding water is, so to speak, 

 " undissolved," and thrown down as a very fine white 

 powder, together with the chalk newly formed by the 

 union of the lime and the carbonic acid. These large 

 tanks are used to allow the fine powder of chalk to settle 

 down and leave the water clear. The brilliantly white 

 chalk sediment accumulates not only on the floor of the 

 tank, but on its sides. Any light which falls on the tank 

 is refracted and reflected from side to floor and from floor 

 to side, and eventually emerges from the tank, a great 

 deal of it having traversed the 20 ft. breadth and depth 

 many times. Most of its red, yellow, and green rays are 

 quenched by the many feet of blue water through which it 

 has passed, and it issues as predominantly blue. This is 

 largely due to the fine reflecting surface furnished by the 

 " white- washed " or chalk-coated floor and sides and the 

 great purity of the white reflecting material — no yellow 

 or brown matter being present to give a greenish tinge to 

 the result. I remember being taken to see " Clark's 



