Hopkinsville Kentuckian., 1910-02-22
PROPERTIES OF ICE.
Why Skating Is Easier When the Tenn
peraturs Is Not Too Low.

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Why cen we skate more easily
when the weather is not too cold [?
Why do our skates "bite" or take
hold of the ice better when the tenn
perature is not far below the freez-
ing point? The explanation of this
simple and universally recognized
fact will enable us also to see the
reason for another, the truth of
which is recognized by every school
boy-namely, that snow will pack
better into snowballs when nearly
at its melting point.
The explanation of both these
facts lies in an almost unique prop
erty of water or, rather, ice, which
causes it to melt or tend to melt
when pressure is applied to it. When
the pressure is relieved, if it is still
below its normal freezing tempera-
ture, it at once solidifes again.
This property is BEAUTIFULLY illus
trated in a familiar experiment in
which a large cake of ice is used,
supported at each end. If loop of
fine wire is passed around it and
a weight attached to the end it will
soon be noted that the wire is slow
ly cutting its way through the ice,
but curiously enough without caus-
ing any division of the cake, for the
ice is melted into water by the
pressure directly beneath the wire,
which water at once freezes into ice
again above the wire, 80 that per-
haps after an hours time the wire
will have cut its way completely
through the cake, leaving it, how
ever, as solid and whole as ever.
In the case of the skater, then,
the weight of his body carried on a
narrow skate may produce a pres
sure on the ice sufficiently great to
melt small groove under the edge
of the runner. This enables it to
take hold of the ice more readily,
and, furthermore, the water acts as
a lubricant on the ice and makes it
slippery. Since considerable pres
sure is necessary to produce melting
even when the temperature is only
a degree or so below the normal
melting point, this effect will not
take place on a very cold day, for,
as is well known, very cold ice is not
slippery. The same explanation
holds for snowballs. But the pres
sure developed in this case is much
less, so that melting enough to
"pack" well will not occur unless
the snow is nearly warm enough to
melt anyway.
This effect is noted on a grand
scale in the case Of glaciers, which
flow slowly down a mountain side
much as if ice was a vicious fluid.
The great pressure developed along
the bottom as well as at the turns
and other points causes a slight
melting, which is Immediately fol
lowed by freezing again as soon as
the pressure is relieved. This en-
ables the ice to adapt itself to the
shape of the valley as it proceeds on
its way.
Ice is one of the few substances
that show this effect of melting un-
der pressure, and it does so because
of the fact that water expands on
freezing. The great majority of
substances contract on solidifying
and hence would show the reverse
effect-that is, pressure applied to
them in the molten state would tend
to cause sohdifcation.
This is one reason for believing
that the interior of our earth as a
whole is solid rather than molten,
for, while we believe that the tenn
perature of the interior may be
much more than enough to melt
rock under ordinary conditions
and indeed volcanoes show that
there is plenty of molten rock not
so very far below the surface-the
enormous pressures that exist deep
down in the earth are enough to
keep the material in a solid condi-.
tion.-Minneapolis Journal.