100 THE FORMS OF WATER IN 



constitute one of the most beautiful features of the 

 higher crevasses. 



253. How are they produced? Evidently by the 

 thawing of the snow. But why, when once thawed, 

 should the water freeze again to solid spears? Yon 

 have seen icicles pendent from a house-eave, which 

 have been manifestly produced by the thawing of the 

 snow upon the roof. If we understand these, we shall 

 also understand the vaster stalactites of the Alpine 

 crevasses. 



254. Gathering up such knowledge as we possess, 

 and reflecting upon it patiently, let us found on it, if 

 we can, a theory of icicles. 



255. First, then, you are to know that the air of our 

 atmosphere is hardly heated at all by the rays of the 

 sun, whether visible or invisible. The air is highly 

 transparent to all kinds of rays, and it is only the 

 scanty fraction to which it is not transparent that ex- 

 pend their force in warming it. 



256. Not so, however, with the snow on which the 

 sunbeams fall. It absorbs the solar heat, and on a 

 sunny day you may see the summits of the high Alps 

 glistening with the water of liquefaction. The air above 

 and around the mountains may at the same time be 

 many degrees below the freezing point in temperature. 



257. You have only to pass from sunshine into shade 

 to prove this. A single step suffices to carry you from 

 a place where the thermometer stands high to one 



