Ice and its Natural History 261 



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 pre- 

 sented is one of very great interest. The illustration, Fig. 4, 

 shows this etching on a buttress in the grotto of the Morter- 

 atsch 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, however, 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 diffi- 

 culty 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 breathes on the polished surface of ice and develops 

 the figure of the ice by the rime which is deposited on it. 

 As rime always settles by preference on sharp edges, it seeks 



