tion, it enters on a role
totally different from that followed by the fluid material.

Beginning its descent to the earth in a snowflake, the little mass
falls slowly, so that when it comes against the earth the blow which
it strikes is so slight that it does no effective work. In the state
of snow, even in the separate flakes, the frozen water contains a
relatively large amount of air. It is this air indeed, which, by
dividing the ice into many flakes that reflect the light, gives it the
white colour. This important point can be demonstrated by breaking
transparent ice into small bits, when we perceive that it has the hue
of snow. Much the same effect is given where glass is powdered, and
for the same reason.

As the snowflakes accumulate layer on layer they imbed air between
them, so that when the material falls in a feathery shape--say to the
depth of a foot--more than nine tenths of the mass is taken up by the
air-containing spaces. As these cells are very small, the circulation
in them is slight, and so the layer becomes an admirable
non-conductor, having this quality for the same reason that feathers
have it--i.e., because the cells are small enough to prevent the
circulation of the air, so that the heat which passes has to go by
conduction, and all gases are very poor conductors. The result is that
a snow coating is in effect an admirable blanket. When the sun shines
upon it, much of the heat is reflected, and as the temperature does
not penetrate it to any depth, only the superficial part is melted.
This molten water takes up in the process of melting a great deal of
heat, so that when it trickles down into the mass it readily
refreezes. On the other hand, the heat going out from the earth, the
store accumulated in its superficial parts in the last warm season,
together with the small share which flows out from the earth's
interior, is held in by this blanket, which it melts but slowly. Thus
it comes about that in regions of long-enduring snowfall the ground,
though frozen to the depth of a foot or more at the time when the
accumulation took place, may be thawed out and so far warmed that the
vegetation begins to grow before the protecting envelope of snow has
melted away. Certain of the early flowers of high latitudes, indeed,
begin to blossom beneath the mantle of finely divided ice.

In those parts of the earth which for the most part receive only a
temporary coating of snow the effect of this covering is