does
the power of the stone to conduct heat vary in different stones of the
same kind or variety, as already explained, but there is seen a
remarkable difference in value, according to the spot on which the heat
is applied, so that on one stone there is often seen a conductivity
varying between 0-15 to 4-70.
This
is owing to the differences of expansion due to the temporary
disturbance of its crystalline structure, brought about by the applied
heat. This will be evident when heat is applied on the axes of the
crystal, on their faces, angles, lines of symmetry, etc., etc., each
one of which gives different results, not only as to value in
conductivity, but a result which varies in a curious degree, out of all
proportion to the heat applied. In many cases a slight diminution in
applied heat gives a greater conductivity, whilst in others a slight
rise in the temperature of the heat destroys its conductivity
altogether, and renders the stone quite opaque to heat-rays.
This
anomaly is due entirely to the alteration of crystalline structure,
which, in the one case, is so changed by the diminution in heat as to
cause the crystals to be so placed that they become diathermal, or
transparent to heat-rays ; whilst, in the other instance, the crystals
which so arrange themselves as to be diathermal are, by a slightly
increased temperature, somewhat displaced, and reflect, or otherwise
oppose the direct passage of heat-rays, which, at the lower
temperature, obtained free passage.
Tims
certain stones become both opaque and diathermal, and as the heat is
caused to vary, so do they show the complete gamut between the two
extremes of total opacity and complete transparency to heat-rays.
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