e 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.

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.

CHAPTER X.

PHYSICAL PROPERTIES.

H--MAGNETIC AND ELECTRIC INFLUENCES.

The word "electricity" is derived from the Greek "elektron," which was
the name for amber, a mineralised resin of extinct pine-trees. It was
well-known to the people of pre-historic times; later to the early
Egyptians, and, at a still later date, we have recorded how Thales--the
Greek philosopher, who lived about the close of the 7th Century B.C.,
and was one of the "seven wise men"--discovered the peculiar property
which we call "electricity" by rubbing dry silk on amber.

Many stones are capable of exhibiting the same phenomenon, not only by
friction, as in Thales's experiment, but also under the influence of
light, heat, magnetism, chemical action, pressure, etc., and of holding
or retaining this induced or added power for a long or short period,
according to conditions and environment.

If a small pith ball is suspended from a non-conducting support, it
forms a simple and ready means of testing the electricity in a stone.
According to whether the ball is repelled or attracted, so is the
electricity in the stone made evident, though the electroscope gives the
better results. By either of these methods it will be found that some of
the stones are more capable of giving and receiving charges of
electricity than are others; also that some are charged throughout with
one kind only, either positive or negative, whilst others have both,
becoming polarised electrically, having one portion of their substance
negative, the other positive. For instance, amber, as is well known,
produces negative electricity under the influence of friction, but in
almost all cut stones, other than amber, the electricity produced by the
same means is positive, whereas in the _uncut_ stones the electricity is
negative, with the exception of the diamond, in which the electricity is
positive.

When heated, some stones lose their electricity; others develop it,
others have it reversed, the positive becoming negative and vice versa;