298 In and Aroiind the Morteratsch Glacier : 



water produced by melting may find, in the interstices so 

 formed between the grains, gutters for efficient drainage. 

 The proportion of radiation so received from the sky by the 

 walls of the grotto diminishes rapidly as the distance from 

 the entrance increases and the occurrence of continuous 

 white ice diminishes at the same rate. At the entrance of 

 the grotto it is only on the roof itself or in its immediate 

 neighbourhood that continuous clear ice is to be seen, and 

 this depends on the fact that these surfaces are by their 

 position protected from direct radiation from the sky, and are 

 exposed only to the much less powerful radiation of the 

 ground in front of the grotto. 



The delineation of the grain on the surface of the walls of 

 the grotto ceases at a distance of four or five metres from the 

 entrance. In certain places, however, where the ice is directly 

 or normally exposed to the light from the entrance, the grain 

 may be beautifully delineated on it. even though it may be 

 half-way up the grotto. Fig. 9, from a photograph made 

 1 9th September 1907, illustrates the revelation of the grain 

 of the ice on the outer end of one of the ice-walls which 

 receives direct daylight ; this, however, is much subdued by 

 the distance, about fifteen metres, of the ice-face from the 

 entrance. This attenuated daylight acts like a very dilute 

 acid in etching. The effect corresponding to the strong acid 

 is the profound disintegration of the ice on the surface of the 

 glacier produced by full exposure to the direct rays of the 

 sun and the hemisphere of the sky. 



Granular Features of the Ice in Winter, 



The winter visits were particularly interesting. The con- 

 dition of the Morteratsch grotto turned out to be different in 

 each of the last four winters in which I visited it. Fig. 7 

 represents it as photographed in January 1907. At that date 

 the most striking as well as the most picturesque feature was 

 the abundant deposit of rime or hoar-frost which penetrated 

 into the innermost parts of the grotto. It covered the ice 

 completely on the roof and on the walls down to a height of 



