corresponding with the directions of the lines of magnetic force, exactly as will iron filings round the two poles of a magnet.
From
this it will clearly be seen how simple a matter it is to isolate the
topaz, tourmaline, and all the pyro-electric stones from the
nou-pyro-electric, for science has not as yet been able to give to
spurious stones these same electric properties, however excellent some
imitations may be in other respects. Further, almost all minerals lose
their electricity rapidly on exposure to atmospheric influences, even
to dry air ; the diamond retains it somewhat longer than most stones,
though the sapphire, topaz, and a few others retain it almost as long
again as the diamond, and these electric properties are some of the
tests which are used in the examination of precious stones.
Those stones which show electricity on the application of pressure are such as the fluorspar, calcite, and topaz.
With
regard to magnetism, the actual cause of this is not yet known with
certainty. It is, of course, a self-evident fact that the magnetic iron
ore, which is a form of peroxide, commonly known as magnetite, or
lodestone, has the power of attracting a magnet when swinging
free, or of being attracted by a magnet, to account for which many
plausible reasons have been advanced. Perhaps the most reasonable and
acceptable of these is that this material contains molecules which have
half their substance positively and the other half negatively
magnetised.
Substances so composed, of which magnets are an
