218 
THE FORMATION OF GLACIERS. 
Such a breaking-up of the ice is always caused 
by the enlargement of the open spaces produced 
by the elevated temperature of the air-bubbles, 
these spaces being necessarily more or less par- 
allel with one another, and vertical in their posi- 
tion, owing to the natural tendency of the air- 
bubbles to work their way upward till they reach 
the surface, where they escape. A sheet of ice, 
of this kind, floating upon water, dissolves in the 
same manner, melting wholly from the surface, 
if the process be sufficiently rapid, or falling to 
pieces, if the air-bubbles are gradually raised in 
their temperature sufficiently to render the whole 
mass cavernous and incoherent. If we now com- 
pare these facts with what is known of the struc- 
ture of land-ice, we shall see that the mode of 
formation in the two cases differs essentially. 
Land-ice, of which both the ice-fields of the 
Arctics and the glaciers consist, is produced by 
the slow and gradual transformation of snow into 
ice ; and though the ice thus formed may eventu- 
ally be as clear and transparent as the purest 
pond- or river-ice, its structure is nevertheless en- 
tirely distinct. We may compare these different 
processes during any moderately cold winter in 
the ponds and snow-meadows immediately about 
us. We need not join an Arctic exploring expe- 
dition, nor even undertake a more tempting trip 
to the Alps, in order to investigate these phenom- 
