1908] on Ice and its Natural History. 263 



articulation 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 crystallograpliically oriented lamina 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 prolonged 

 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 of direct sky-light for the disarticulation of 

 glacier ice into its constituent grains is very well seen in the arti- 

 ficial grottos which are maintained at easily accessible parts of most 

 popular glaciers. One of the galleries in the grotto of the Morter- 

 atsch glacier is represented in Fig. 3. It is from a photograph 

 which I took in January 1907. The hoar-frost on the roof and the 

 sharp line where it ceases on the walls are well shown. The white 

 object on the left is a stone working its way out through the ice. 

 Many of these were visible in the body of the ice. The thickness of 

 the layer of completely disarticulated ice is so small that it is hardly 

 noticed, and the whole grotto appears 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 linear figures. These are most strongly marked near the 

 opening, and they extend as far as 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. The illustration. Fig. 4, shows this etching on a 

 buttress in the grotto of the Morteratsch glacier, taken in September 

 1907. 



After the autumnal equinox very little melting of ice takes place, 

 and by the end of October it has, as a rule, ceased entirely. The 

 etched figures on the walls of the entrance of the grotto, which were 

 developed during summer, disappear quickly with the arrival of winter. 

 But the winter brings with it another means of delineation of the 

 grain which does not depend on solar radiation. Even at the lowest 

 of winter temperatures, the atmosphere contains vapour of water, 

 which it is prepared to relinquish under the same conditions as those 

 under which dew is formed in summer. In the Alpine winter, how- 

 ever, it is deposited not as dew^ but as rime, that is, not as water but 

 as ice. It is well known that very fine etching on a polished surface, 

 which can with difficulty be seen without assistance, at once becomes 

 visible if the surface be breathed on. In winter, the walls and roof 

 of the grotto are cold, dry, smooth and polished like glass. The 

 winter air entering from without and circulating in the grotto hreathes 

 on the poHshed surface of ice and develops the figure of the ice by the 

 rime which is deposited on it. As rime always settles by preference 



