The Size of the Ice-grain in Glaciers 231 



the disintegration of the crystal under the absorption of 

 radiant heat. Similar and complementary features are ob- 

 served when ice is generated from an existing crystal under 

 the dissipation of heat. To do justice to them, however, 

 would require the services of a skilful, patient and resourceful 

 artist. 



The disarticulating and analysing action of the sun's rays 

 is not accomplished without the selection and expenditure 

 of energy. Accordingly we observe that one grain protects 

 another. The disarticulation into separate grains, although 

 very thorough near the surface of a glacier, does not penetrate 

 far. A stroke or two with an ice-axe reveals the fresh blue 

 ice. The analysis of the individual grain into crystallo- 

 graphically oriented laminae can be particularly well studied 

 in the Mergelin See. It is only the grains that are exposed 

 to the sky, and above water, that are so analysed ; and pro- 

 longed exposure of this kind reduces a grain to the last stage 

 of dilapidation. The grains beneath the surface, whether of 

 ice or water, are almost completely unattacked. 



The importance, or rather the necessity, of direct sky- 

 light for the disarticulation of glacier-ice into its constituent 

 grains is very well seen in the artificial grottos which are 

 maintained at easily accessible parts of most popular glaciers. 

 The thickness of the layer of completely disarticulated ice is 

 so small that it is hardly noticed, and the whole grotto ap- 

 pears to be cut out of pure blue ice. If the observer, on 

 penetrating for a few paces, turns round and looks outwards, 

 he sees the surface of the ice-walls of the grotto etched with 

 strange line-figures. These are most strongly marked near 

 the opening, and they cease exactly at the spot where the 

 last ray of direct sky-light strikes the ice. The lines so 

 developed are formed by the intersection of the surface of the 

 ice-wall of the cave with the separating surfaces of contiguous 

 ice-grains. The photographic picture thus presented is one 

 of very great interest. 



It is only perfectly pure water, received directly as it flows 

 from the still, that can be frozen into homogeneous glass-like 

 ice. All natural ice proceeds from impure water. 



