neness. This is especially appreciated when it is
remembered that so far science has been unable (except in very rare
instances of no importance) to manufacture any stone of the same colour
as the genuine and at the same time of the same specific gravity. Either
the colour and characteristics suffer in obtaining the required weight
or density, or if the colour and other properties of an artificial stone
are made closely to resemble the real, then the specific gravity is so
greatly different, either more or less, as at once to stamp the jewel as
false. In the very few exceptions where chemically-made gems even
approach the real in hardness, colour, specific gravity, &c., they cost
so much to obtain and the difficulties of production are so great that
they become mere chemical curiosities, far more costly than the real
gems. Further, they are so much subject to chemical action, and are so
susceptible to their surroundings, that their purity and stability
cannot be maintained for long even if kept airtight; consequently these
ultra-perfect "imitations" are of no commercial value whatever as
jewels, even though they may successfully withstand two or three tests.

CHAPTER IX.

PHYSICAL PROPERTIES.

G--HEAT.

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, b