THE COLOUR OF ICE 47 



in it, from dead or living vegetable matter or from iron 

 salts, the colour is green. There are two readily available 

 exhibitions of the blue colour of water with which anyone 

 may satisfy himself on the subject. The first is that of 

 the tanks of some of the water-supplying companies, such 

 as those to be seen from the railway near Caterham. 

 These tanks are cube-like reservoirs, twenty feet deep. 

 They are used to soften the water by precipitating the 

 chalk dissolved in it, and the deposit of white chalk lines 

 the bottom and sides of the tanks whilst the water itself 

 becomes of crystalline purity. Even on the most cloudy 

 days these tanks stand out in the scene as patches of 

 brilliant cobalt blue. A simpler case is that of the large 

 brilliantly white porcelain baths now provided in bath- 

 rooms. If the room is well lit from above by strong sun- 

 light, and has a white wall, and the bath is well filled with 

 good clear water, the latter appears strongly blue, any 

 wave or rippling of the surface appearing as bands of 

 bright blue. In this case the light is reflected to and fro 

 by the sides of the bath, and an effect like that of the blue 

 grotto of Capri is obtained. If the water should appear 

 at all green, it is due to yellow-coloured impurity in the 

 water, or in the porcelain, or in the colouring of the sides 

 of the room. 



Liquid oxygen (prepared by modern methods of pro- 

 ducing extreme cold) is also blue. It is not surprising 

 that solid water, which is what we look into in the great 

 chasms in the clear ice of glaciers, should show this colour. 

 Glaciers often, however, appear bluish-green, especially near 

 the surface, or when seen indistinctly at a distance. This 

 is due to fine dust from the atmosphere, which falls con- 

 tinually on the mountain snow, and contains iron, which 

 forms yellow-coloured rust in minute quantity. Some of 

 the dust which falls on the snow of mountains and on 

 the ocean (sinking there to the bottom) is of terrestrial 



