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 hemimorphic, 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 50 deg. F. and 300 deg. F. it assumes
electric polarity, becoming electrified positively at one end or pole
and negatively at the opposite pole. If it is suspended on a silken
thread from a glass rod or other non-conducting support in a similar
manner to the pith ball, the tourmaline will be found to have become an
excellent magnet. By testing this continually as it cools there will
soon be perceived a point which is of extreme delicacy of temperature,
where the magnetic properties are almost in abeyance. But as the
tourmaline cools yet further, though but a fraction of a degree, the
magnetic properties change; the positive pole becomes the negative, the
negative having changed to the positive.

It is also interesting to note that if the tourmaline is not warmed so
high as to reach a temperature of 50 deg. F., or is heated so strongly as to
exceed more than a few degrees above 300 deg. F., then these magnetic
properties do not appear, as no polarity is present. This polarity, or
the presence of positive and negative electricity in one stone, may be
strikingly illustrated in a very simple manner:--If a little sulphur and
red-lead, both in fine powder, are shaken up together in a paper or
similar bag, the moderate friction of particle against particle
electrifies both; one negatively, the other positively. If, then, a
little of this now golden-coloured mixture is gently dusted over the
surface of the tourmaline or other stone possessing electric polarity, a
most interesting change is at once apparent. The red-lead separates
itself from the sulphur and adheres to the negative portion of the
stone, whilst the separated sulphur is at once attracted to the positive
end, so that the golden-colour