io6 
SCENERY OF SWITZERLAND. 
summers the thickness of the layer of firn increases. 
It is deepest in the higher regions, and thins out 
gradually, until at length ice appears on the actual 
surface, and the firn passes into a glacier. 
Glaciers are in fact rivers of ice, which indeed 
sometimes widen out into lakes. Glacier ice differs 
considerably from firn ice, and the molecular process 
by which the one passes into the other is not yet 
thoroughly understood. Again, if a piece of ice from 
a lake is melted in warm air the surface gradually 
liquefies and the whole remains clear; on the con- 
trary, a piece of compact glacier ice from the deeper 
part of a glacier if similarly treated behaves very 
differently; a number of capillary cracks appear, 
which become more and more evident, and gradually 
the ice breaks up into irregular, angular, crystalline 
fragments. These are known as the “grains du 
glacier” or “Gletscherkorn,” and were first described 
by Hugi* They increase gradually in size, but how 
this growth takes place, and whether they are derived 
from the granules of the firn, is still doubtful. When 
the firn passes into the glacier they may be about 
1 / i inch across; in the middle part of a large glacier 
about the size of a walnut, and at the end 4 or even 
6 inches in diameter. Those at the end of the Rhone 
* Das Wescn der Gletscher, 1842 . 
