ually be as clear and
transparent as the purest pond- or river-ice, its structure is
nevertheless entirely distinct. We may trace these different processes
during any moderately cold winter in the ponds and snow-meadows
immediately about us. We need not join an Arctic exploring expedition,
nor even undertake a more tempting trip to the Alps, in order to
investigate these phenomena for ourselves, if we have any curiosity to
do so. The first warm day after a thick fall of light, dry snow, such as
occurs in the coldest of our winter weather, is sufficient to melt its
surface. As this snow is porous, the water readily penetrates it, having
also a tendency to sink by its own weight, so that the whole mass
becomes more or less filled with moisture in the course of the day.
Daring the lower temperature of the night, however, the water is frozen
again, and the snow is now filled with new ice-particles. Let this
process be continued long enough, and the mass of snow is changed to a
kind of ice-gravel, or, if the grains adhere together, to something like
what we call pudding-stone, allowing, of course, for the difference of
material; the snow, which has been rendered cohesive by the process of
partial melting and regelation, holding the ice-globules together, just
as the loose materials of the pudding-stone are held together by the
cement which unites them.

Within this mass, air is intercepted and held inclosed between the
particles of ice. The process by which snow-flakes or snow-crystals are
transformed into grains of ice, more or less compact, is easily
understood. It is the result of a partial thawing, under a temperature
maintained very nearly at thirty-two degrees, falling sometimes a little
below, and then rising a little above the freezing-point, and thus
producing constant alternations of freezing and thawing in the same mass
of snow. This process amounts to a kind of kneading of the snow, and
when combined with the cohesion among the particles more closely held
together in one snow-flake, it produces granular ice. Of course, the
change takes place gradually, and is unequal in its progress at
different depths in the same bed of recently fallen snow. It depends
greatly on the amount of moisture infiltrating the mass, whether derived
from the melting of its own surface, or from the accumulation of dew or
the falling of rain or mist upon it. The amount of water retained within
the mass will also be greatly affected by the