A Study in the Natural History of Ice 299 



about 1*4 metres above the floor of the grotto where it ceased. 

 Below this line it was completely absent. The deposit of 

 hoar-frost was especially fine in the middle chamber of the 

 grotto, shown in Fig. 7, where it furnished a wonderful picture 

 of ice-flowers. In the galleries it was less abundant. It 

 settled, as may be seen in Fig. 8, in the shell-like cavities of 

 the roof as a comparatively thin coating and in great abund- 

 ance on the sharp ice-edges which separated these cavities. 

 In these cavities the delineation of the grain of the ice 

 produced by the condensation of the hoar-frost upon it was 

 particularly beautiful. The phenomenon was not repeated in 

 the three following winters, and I am not able to give a 

 sufficient reason why it was so abundant in the one winter 

 and was practically absent in the others. 



It is to be remarked that in winter the ice of the walls of 

 the grotto has a temperature below its melting-point, it is hard 

 and polished like glass, and its surface bears no delineation of 

 the grain. 



During this season such delineation is met with naturally 

 in the interior of the grotto only as the result of the con- 

 densation of moisture as hoar-frost, and artificially in the 

 immediate neighbourhood of the lamps used for illumination. 

 Each of these lamps is a small local centre of disarticulation 

 because it supplies not only the heat of convection which 

 raises the temperature of the surface of the ice in its im- 

 mediate neighbourhood to o C, but also the radiant energy 

 which penetrates the surface and produces intergranular 

 melting. The texture of the ice in immediate proximity to 

 such a lamp simulates on a small scale that of the surface of 

 the glacier which is due to the strong light of the sun. When 

 I first visited the Morteratsch grotto only petroleum lamps 

 were used, each of which melted a considerable dome out of 

 the roof. Later these were replaced by acetylene lamps 

 which produced greater wastage on the walls. Now the 

 grotto is illuminated by electric incandescent lights which 

 produce disarticulation but little melting. 



