ly in this manner, their separation being rendered still
more distinct by the accumulation of air-bubbles, which, during a hot
and clear day, may rise from a muddy bottom in great numbers. In
consequence of these occasional collections of air-bubbles, the layers
differ, not only in density and closeness, but also in color, the more
compact strata being blue and transparent, while those containing a
greater quantity of air-bubbles are opaque and whitish, like water
beaten to froth.

A cake of pond-ice, such as is daily left in summer at our doors, if
held against the light and turned in different directions, will exhibit
all these phenomena very distinctly, and we may learn still more of its
structure by watching its gradual melting. The process of decomposition
is as different in fresh-water ice and in land-or glacier-ice and that
of their formation. Pond-ice, in contact with warm air, melts uniformly
over its whole surface, the mass being thus gradually reduces from the
exterior till it vanishes completely. If the process be slow, the
temperature of the air-bubbles contained in it may be so raised as to
form the vertical funnels or tubes alluded to above. By the anastomosing
of these funnels, the whole mass may be reduced to a collection of
angular pyramids, more or less closely united by cross-beams of ice, and
it finally falls to pieces when the spaces in the interior have become
for numerous as to render it completely cavernous. Such a breaking-up of
ice is always caused by the enlargement of the open spaces produces by
the elevated temperature of the air-bubbles, these spaces being
necessarily more or less parallel with one another, and vertical in
their position, 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 compare these facts with what is known of the
structure 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 glaciers
consist, is produced by the slow and gradual transformation of snow into
ice; and though the ice thus formed may event