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PRECIOUS STONES
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ing
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; others
again, when heated, become powerfully magnetic and assume strong
polarity. When electricity develops under the influence of heat, or is
in any way connected with a rising or falling of temperature in a
body, it is called " pyro-electricity," from the Greek word " pyros,"
fire. The phenomenon was first discovered in the tourmaline, and it is
observed, speaking broadly, only in those minerals which are
hemi-morphic, that is, where the crystals have different planes or
faces at their two ends, examples of which are seen in such crystals as
those of axinite, boracite, smithsonite, topaz, etc., all of which are
hemimorphic.
Taking the tourmaline as an example of the pyro-electric minerals, we find that when this is heated to between 503
F. and 800° F. it assumes electric polarity, becoming electrified
positively at one end or pofe and negatively at the opposite pole. If
it is suspended on a silken thread from a glass rod or other
non-conducting
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