<h2>CHAPTER IX.</h2>
<h3>PHYSICAL PROPERTIES.</h3>
<h4><span class="smcap">G—Heat.</span></h4>
<p>Another method of isolating certain stones is by the action of
heat-rays. Remembering our lessons in physics we recall that just as
light-rays may be refracted, absorbed, or reflected, according to the
media through which they are caused to pass, so do heat-rays possess
similar properties. Therefore, if heat-rays are projected through
precious stones, or brought to bear on them in some other manner than by
simple projection, they will be refracted, absorbed, or reflected by the
stones in the same manner as if they were light-rays, and just as
certain stones allow light to pass through their substance, whilst
others are opaque, so do some stones offer no resistance to the passage
of heat-rays, but allow them free movement through the substance,
whilst, in other cases, no passage of heat is possible, the stones being
as opaque to heat as to light. Indeed, the properties of light and heat
are in many ways identical, though the test by heat must in all cases
give place to that by light, which latter is by far of the greater
importance in the judging and isolation of precious stones. It will
readily be understood that in the spectrum the outer or extreme
light-rays at each side are more or less bent or diverted, but those
nearest the<span class="pagenum"><SPAN name="Page_53" id="Page_53">[Pg 53]</SPAN></span> centre are comparatively straight, so that, as before
remarked, these central rays are taken as being the standard of
light-value. This divergence or refraction is greater in some stones
than in others, and to it the diamond, as an example, owes its chief
charm. In just such manner do certain stones refract, absorb, or reflect
heat; thus amber, gypsum, and the like, are practically opaque to
heat-rays, in contrast with those of the nature of fluorspar, rock-salt,
&c., which are receptive. Heat passes through these as easily as does
light through a diamond, such stones being classed as diathermal (to
heat through). So that all diathermal stones are easily permeable by
radiant heat, which passes through them exactly as does light through
transparent bodies.</p>
<p>Others, again, are both single and double refracting to heat-rays, and
it is interesting to note the heat-penetrating value as compared with
the refractive indexes of the stone. In the following table will be
found the refractive indexes of a selection of single and double
refractive stones, the figures for "Light" being taken from a standard
list. The second column shows the refractive power of heat, applied to
the actual stones, and consisting of a fine pencil blowpipe-flame, one
line (the one twelfth part of an inch) in length in each case. This list
must be taken as approximate, since in many instances the test has been
made on one stone only, without possibility of obtaining an average; and
as stones vary considerably, the figures may be raised or lowered
slightly, or perhaps even changed in class, because in some stones the
least stain or impurity may cause the heat effects to be altered greatly
in their<span class="pagenum"><SPAN name="Page_54" id="Page_54">[Pg 54]</SPAN></span> character, and even to become singly or doubly refracting,
opaque or transparent, to heat-rays, according to the nature of the
impurity or to some slight change in the crystalline structure, and so
on.</p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td colspan="2"><i>Selection of Singly refracting stones.</i></td><td colspan="2"><i>Indexes of Rays of</i></td></tr>
<tr><td align='left'></td><td align='left'><span class="smcap">Light.</span></td><td align='left'><span class="smcap">Heat.</span></td></tr>
<tr><td align='left'>Fluorspar</td><td align='left'>1.436</td><td align='left'>4.10</td><td align='left'>varies</td></tr>
<tr><td align='left'>Opal</td><td align='left'>1.479</td><td align='left'>2.10</td><td align='left'>"</td></tr>
<tr><td align='left'>Spinel</td><td align='left'>1.726</td><td align='left'>1.00</td></tr>
<tr><td align='left'>Almandine</td><td align='left'>1.764</td><td align='left'>1.00</td></tr>
<tr><td align='left'>Diamond</td><td align='left'>2.431</td><td align='left'>6.11</td><td align='left'>double</td></tr>
</table></div>
<p><br/><br/></p>
<div class='center'>
<table border="0" cellpadding="4" cellspacing="0" summary="">
<tr><td colspan="2"><i>Selection of Doubly refracting stones.</i></td><td colspan="2"><i>Indexes of Rays of</i></td></tr>
<tr><td align='left'></td><td align='left'>Light.</td><td align='center'>Heat.</td></tr>
<tr><td align='left'>Quartz</td><td align='left'>1.545</td><td align='left'>4.7</td><td align='left'>single and double</td></tr>
<tr><td align='left'>Beryl</td><td align='left'>1.575</td><td align='left'>1.0</td><td align='left'>varies considerably</td></tr>
<tr><td align='left'>Topaz</td><td align='left'>1.635</td><td align='left'>4.1</td><td align='left'> " "</td></tr>
<tr><td align='left'>Chrysoberyl</td><td align='left'>1.765</td><td align='left'>1.1</td><td align='left'> " "</td></tr>
<tr><td align='left'>Ruby</td><td align='left'>1.949</td><td align='left'>5.1</td><td align='left'>single and double</td></tr>
</table></div>
<p>The tourmaline has a light-refractive index of 1.63, with a heat index
of none, being to heat-rays completely opaque.</p>
<p>The refractive index of gypsum is 1.54, but heat none, being opaque.</p>
<p>The refractive index of amber is 1.51, but heat none, being opaque.</p>
<p>In some of the specimens the gypsum showed a heat-penetration index of
0.001, and amber of 0.056, but mostly not within the third point. In all
cases the heat-penetration and refraction were shown by electric
recorders. These figures are the average of those obtained from tests
made in some cases on several stones of the same kind, and also on
isolated specimens. Not only<span class="pagenum"><SPAN name="Page_55" id="Page_55">[Pg 55]</SPAN></span> 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.</p>
<p>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.</p>
<p>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.</p>
<p>Thus 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.<span class="pagenum"><SPAN name="Page_56" id="Page_56">[Pg 56]</SPAN></span></p>
<p>For the purpose under consideration, the temperature of the pencil of
heat applied to the stones in their several portions was kept constant.
It will be seen, therefore, that no great reliance can be placed on the
heat test as applied to precious stones.</p>
<hr style="width: 65%;" />
<p><span class="pagenum"><SPAN name="Page_57" id="Page_57">[Pg 57]</SPAN></span></p>
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