Could we melt the whole of the ice in this manner? Again the
answer is "yes." But the pressure must be very great. If we
assume that all the heat is obtained at the expense of the
sensible heat of the ice, the cooling must be such as to supply
the latent heat of the whole mass of water produced. However, the
latent heat diminishes as the melting point is lowered, and at a
rate which would reduce it to nothing at about 18,000
atmospheres. Mousson, operating on ice enclosed in a conducting
cylinder and cooled to -18° at starting
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appears to have obtained very complete liquefaction. Mousson must
have attained a pressure of at least an amount adequate to lower
the melting point below -18°. The degree of liquefaction actually
attained may have been due in part to the passage of heat through
the walls of the vessel. He proved the more or less complete
liquefaction of the ice within the vessel by the fall of a copper
index from the top to the bottom of the vessel while the pressure
was on.
I have here a simple way of demonstrating to you the fall of
temperature attending the compression of ice. In this mould,
which is strongly made of steel, lined with boxwood to diminish
the passage of conducted heat, is a quantity of ice which I
compress when I force in this plunger. In the ice is a
thermoelectric junction, the wires leading to which are in
communication with a reflecting galvanometer. The thermocouple is
of copper and nickel, and is of such sensitiveness as to show by
motion of the spot of light on the screen even a small fraction
of a degree. On applying the pressure, you see the spot of light
is displaced, and in such a direction as to indicate cooling. The
balancing thermocouple is all the time imbedded in a block of ice
so that its temperature remains unaltered. On taking off the
pressure, the spot of light returns to its first position. I can
move the spot of light backwards and forwards on the screen by
taking off and putting on the pressure. The effects are quite
instantaneous.
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The fact last referred to is very important. The ice, in fact, is
as it were automatically turned to water. It is not a matter of
the conduction of heat from point to point in the ice. Its own
sensible heat is immediately absorbed throughout the mass. This
would be the theoretical result, but it is probable that owing to
imperfections throughout the ice and failure in uniformity in the
distribution of the stress, the melting would not take place
quite uniformly or homogeneously.
Before applying our new ideas to skating, I want you to notice a
fact which I have inferentially stated, but not specifically
mentioned. Pressure will only lead to the melting of ice if the
new melting point, _i.e._ that due to the pressure, is below the
prevailing temperature. Let us take figures. The ice to start
with is, say, at -3° C. Suppose we apply such a pressure to this
ice as will confer a melting point of -2° C. on it. Obviously,
there will be no melting. For why should ice which is at -3° C.
melt when its melting point is -2° C.? The ice is, in fact,
colder than its melting point. Hence, you note this fact: The
pressure must be sufficiently intense to bring the melting point
below the prevailing temperature, or there will be no melting;
and the further we reduce the melting point by pressure below the
prevailing temperature, the more ice will be melted.
We come at length to the object of our remarks I don't know who
invented skating or skates. It is said that in the thirteenth
century the inhabitants of
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England used to amuse themselves by fastening the bones of an
animal beneath their feet, and pushing themselves about on the
ice by means of a stick pointed with iron. With such skates, any
performance either on inside or outside edge was impossible. We
are a conservative people. This exhilarating amusement appears to
have served the people of England for three centuries. Not till
1660 were wooden skates shod with iron introduced from the
Netherlands. It is certain that skating was a fashionable
amusement in Pepys' time. He writes in 1662 to the effect: "It
being a great frost, did see people sliding with their skates,
which is a very pretty art." It is remarkable that it was the
German poet Klopstock who made skating fashionable in Germany.
Until his time, the art was considered a pastime, only fit for
very young or silly people.