224 ICE ITS FORMS AND FUNCTIONS. 



down into the glacieres, and the lighter warm air of summer 

 cannot on ordinary principles of gravitation dislodge it, so 

 that heat is very slowly spread in the caves \ and even 

 when some amount of heat does reach the ice, the latter 

 melts but slowly, for ice absorbs 60 C. of heat in melting ; 

 and thus when ice is once formed, it becomes a material 

 guarantee for the permanence of cold in the cave. For this 

 explanation to hold good it is necessary that the level at 

 which the ice is found should be below the level of the 

 entrance to the cave; otherwise the mere weight of the 

 cold air would cause it to leave its prison as soon as the 

 spring warmth arrived. In every single case that has come 

 under my observation this condition has been emphati- 

 cally fulfilled. It is necessary also that the cave should be 

 protected from direct radiation, as the gravitation of cold 

 air has nothing to do with resistance to that powerful means 

 of introducing heat. A third and very necessary condition 

 is, that the wind should not be allowed access to the cave ; 

 for if it were, it would infallibly bring in heated air in spite 

 of the specific weight of the cold air stored within. There 

 can be no doubt, too, that the large surfaces which are avail- 

 able for evaporation have much to do with maintaining a 

 somewhat lower temperature of the place where the air 

 occurs. Another great advantage which some glacieres 

 possess must be borne in mind namely, the collection of 

 snow at the bottom of the pit in which the entrance lies. 

 This snow absorbs in the course of melting all which strikes 

 down by radiation, or is drawn down by accidental turns 

 of the wind ; and the snow-water thus forced into the cave 

 will at any rate not seriously injure the ice." So much 

 for ice as it appears on the land ; let us next turn to its 

 aspects and functions on the water. 



The last and most obvious division of our subject, per- 



