Solar Energy in Relation to Ice. 149 



does dark heat emanating from a less-intense source. A 

 large part of the heat from the more-intense source passes 

 through the ice-lens ; while, in the case of the heat of less- 

 intense origin, nearly the whole of it is intercepted by the 

 ice within the first eighth of an inch of the lens next the 

 source of heat, and soon begins to melt it. The capacity for 

 melting ice or snow, which most of the solar rays possess, is, 

 therefore, inversely proportional to the refrangibility of these 

 rays ; or, what amounts to the same thing, that capacity 

 is inversely proportional to the frequency of the ethereal 

 undulations to which these heat rays are due. 



One may further illustrate this important principle by 

 reference to phenomena that may easily be studied on any 

 sunny day after a fall of snow : — N'ear to any stone the snow 

 melts, because of the heat radiated from the stone : far from 

 that stone the snow, if clean, remains unmelted. Drop a 

 twig, or a leaf, on to snow, or, better still, sprinkle a little 

 dust on it, and in a few minutes the sunshine will cause these 

 to radiate heat to the snow around, and thus to melt it. 

 On the Swiss glaciers (as was illustrated by Dr Tempest 

 Anderson's beautiful views when this paper was read) sand 

 and small stones quickly melt the ice and snow around ; and 

 it is probably the action of the Sun upon the fine particles 

 of rock blown on to the Alpine snows at high levels that 

 brings about the conversion of this snow into firn or neve. 

 Dirty snow and ice melt quickly in sunshine, while clean 

 snow and ice hardly melt at all. Around the well-known 

 glacier tables so commonly seen on glaciers, phenomena due 

 to the same principle may be seen. Sunshine falls on the 

 stone " table " and warms it, but the heat does not readily 

 pass downwards through the mass, if it is large, so the ice 

 beneath it is not melted, except to a very small extent on the 

 south side. But dark heat is abundantly radiated from the 

 sides of the stone, and notably so from the side facing the 

 midday Sun. Therefore much of the ice to the south of 

 the stone is melted by radiant heat, less is melted on the 

 east and the west, and least of all on the north. So the 

 stone eventually comes to occupy a pedestal, whose position 

 is in the northern focus of an elliptical depression, which has 



