ut those
nearest the 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.

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 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.

_Selection of Singly refracting stones._ _Indexes of Rays of_
LIGHT. HEAT.
Fluorspar 1.436 4.10 varies
Opal 1.479 2.10 "
Spinel 1.726 1.00
Almandine 1.764 1.00
Diamond 2.431 6.11 double

_Selection of Doubly refracting stones._ _Indexes of Rays of_
LIGHT. HEAT.
Quartz 1.545