Water and air, each being
transparent when separate, become opaque when intimately mingled; the
reason being that the inequalities of refraction break up and scatter
every ray of light. Thus, clouds cast a shadow; so does steam; so does
foam: and the same elements take a still denser texture when combined
as snow. Every snow-flake is permeated with minute airy chambers, among
which the light is bewildered and lost; while from perfectly hard and
transparent ice every trace of air disappears, and the transmission
of light is unbroken. Yet that same ice becomes white and opaque when
pulverized, its fragments being then intermingled with air again,--just
as colorless glass may be crushed into white powder. On the other
hand, Professor Tyndall has converted slabs of snow to ice by regular
pressure, and has shown that every Alpine glacier begins as a snow-drift
at its summit, and ends in a transparent ice-cavern below. "The blue
blocks which span the sources of the Arveiron were once powdery snow
upon the slopes of the Col du Geant."
The varied and wonderful shapes assumed by snow and ice have been best
portrayed, perhaps, by Dr. Kane in his two works; but their resources of
color have been so explored by no one as by this same favored Professor
Tyndall, among his Alps. It appears that the tints which in temperate
regions are seen feebly and occasionally, in hollows or angles of fresh
drifts, become brilliant and constant above the line of perpetual snow,
and the higher the altitude the more lustrous the display.
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