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
