64 PHYSIOGRAPHY. [chap. 



in the snow not only confers upon it this valuable propert)', 

 but it also gives the snow its opaque white appearance, so 

 different from the transparency of common ice. The light, 

 instead of penetrating the snow, is thrown back from the 

 ice-walls of each little air-cell or cavity, and thus becomes 

 scattered, the snow losing its transparency ; just as the 

 foam of the sea becomes opaque white, by the light being 

 scattered from the particles of water into M-hich a wave 

 is broken. 



When snow falls upon a mountain in winter, it may lie 

 there unmelted uwtil the warmth of summer returns to 

 thaw it. But, if the mountain be very high, the summer- 

 heat may never be strong enough to melt all the ice on its 

 top, and the top will therefore be enveloped in perpetual 

 snow. A line drawn at the level above which the snow never 

 melts is called the snow-line. On the north side of the 

 Himalaya Mountains this line is 16,600 feet high; that is 

 to say, all the snow which falls below this height is melted 

 in summer, but all above remains unmelted. In the Andes 

 of Peru the limit of perpetual snow is about 15,500 

 feet ; but in passmg northwards or southwards from these 

 hot regions, we expect to find the snow-line descending ; in 

 the Swiss Alps, for example, it comes down to about 8,500 

 feet above the sea. Still farther north it reaches yet lower, 

 and, in the Arctic regions, descends to the very sea-level ; 

 the winter's accumulation of snow is never completely 

 melted by the summer-sun, and the snow consequently lies 

 on the ground all the year round. 



Snow is not the only solid form in which atmospheric 

 moisture is precipitated. Occasionally, during a storm, it 

 takes the shape of hail^ which consists of hard masses of 

 ice varying in size from the smallest shot to pieces several 

 inches in diameter. These hailstones are in some cases 



