You perceive from the table that while the friction of brass or
aluminium on glass is quite independent of the weight used, that
of brass or aluminium on ice depends in some way upon the weight,
and falls in a very marked degree when the weight is heavy. Now,
I think that if we had been on the look out for any abnormality
in the friction of hard substances on ice, we would have rather
anticipated a variation in the
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other direction. We would have, perhaps, expected that a heavy
weight would have given rise to the greater friction. I now turn
to the explanation of this extraordinary result.
You are aware that it requires an expenditure of heat merely to
convert ice to water, the water produced being at the temperature
of the ice, _i.e._ at 0° C., from which it is derived. The heat
required to change the ice from the solid to the liquid state is
the latent heat of water. We take the unit quantity of heat to be
that which is required to heat 1 kilogram of water 1° C. Then if
we melt 1 kilogram of ice, we must supply it with 80 such units
of heat. While melting is going on, there is no change of
temperature if the experiment is carefully conducted. The melting
ice and the water coming from it remain at 0° C. throughout the
operation, and neither the thermometer nor your own sensations
would tell you of the amount of heat which was flowing in. The
heat is latent or hidden in the liquid produced, and has gone to
do molecular work in the substance. Observe that if we supply
only 40 thermal units, we get only one-half the ice melted. If
only 10 units are supplied, then we get only one eighth of a
kilogram of water, and no more nor less.
I have ventured to recall to you these commonplaces of science
before considering a mode of melting ice which is less generally
known, and which involves no supply of heat on your part. This
method involves for its
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understanding a careful consideration of the thermal properties
of water in the solid state.
It must have been observed a very long time ago that water
expands when it freezes. Otherwise ice would not float on water;
and, what is perhaps more important in your eyes, your water
pipes would not burst in winter when the water freezes therein.
But although the important fact of the expansion of water on
freezing was so long presented to the observation of mankind, it
was not till almost exactly the middle of the last century that
James Thomson, a gifted Irishman, predicted many important
consequences arising from the fact of the expansion of water on
becoming solid. The principles lie enunciated are perfectly
general, and apply in every case of change of volume attending
change of state. We are here only concerned with the case of
water and ice.
James Thomson, following a train of thought which we cannot here
pursue, predicted that owing to the fact of the expansion of
water on becoming solid, pressure will lower the melting point of
ice or the freezing point of water. Normally, as you are aware,
the temperature is 0° C. or 32° F. Thomson said that this would
be found to be the freezing point only at atmospheric pressure.
He calculated how much it would change with change of pressure.
He predicted that the freezing point would fall 0.0075 of a
degree Centigrade for each additional atmosphere of pressure
applied to the water. Suppose,
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