302 THE BEDSHIEL KAIMS 



we must notice here. Clean ice and snow are both, of 

 themselves, transparent to most of the heat rays of the Sun, 

 in very much the same way as glass is transparent to the 

 light rays from that source. That is to say, a large part of 

 the heat coming from the Sun goes straight through clean 

 ice without warming it. But, if the ice contains foreign 

 particles of any kind, even fine dust, the Sun's heat-rays 

 do warm these, and the heat taken up by these foreign 

 bodies connected with the ice is given off in a form which 

 is capable of melting ice and snow, which the heat of 

 different quality, coming direct from the Sun, can only do 

 to a very limited extent. Snow and ice, therefore, are 

 melted only in an indirect way by the Sun. Even to melt 

 a little of the ice by this roundabout method, an enormous 

 quantity of sunheat is required, and the temperature of the 

 ice or the snow cannot, of course, notwithstanding all this, 

 be raised above 32° F. Now, air is like ice in respect 

 to sunheat. Dry and clean air is not warmed in the least 

 by sunheat, be the temperature in the sunshine what it may. 

 To warm the air the Sun has first to warm something else, 

 which something else, in its turn, is rendered capable of 

 warming the air. If ice and snow are not warmed by sun- 

 heat, and air is warmed only by radiation from a bare 

 surface, it is obvious that, be the temperature of the sunheat 

 what it may, the temperature of the air in contact with a 

 snow-covered surface cannot rise above the freezing point. 

 It follows also, from this principle, that when once any 

 given area is covered with snow, its boundaries tend to 

 increase, because the chilled air on the snow-clad surface 

 freezes any aqueous vapour near it into snow, instead of 

 condensing it into rain. Furthermore, as Dr Croll has well 

 pointed out, there must always be dense fogs along the 

 margin of any warm sea-current flowing near snow-covered 

 land, and these fogs also conduce to a lowering of tem- 

 perature, by cutting off the sunshine. 



The foregoing remarks are made with the object of 

 showing that glacial conditions may be set up and kept 

 going as a consequence of purely geographical conditions. 

 The nature of these may be summarised, in general terms, 

 as the result of two factors, one of which is that there shall 



