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MAGNETIC AND ELECTRIC INFLUENCES 61
example,
may be made the means of magnetising other substances by friction,
without they themselves suffering any loss ; but it is not all
substances that will respond to the magnet. For instance, common iron
pyrites, FeS2, is unresponsive, whilst the magnetic pyrites, which varies from 5FeS, Fe2S3, to 6FeS, Fe2S3,
and is a sulphide of iron, is responsive both positively and
negatively. Bismuth and antimony also are inactive, whilst almost all
minerals containing even a small percentage of iron will deflect the
magnetic needle, at least under the influence of heat. So that from the
lodestone—the most powerfully magnetic mineral known—to those minerals
possessing no magnetic action whatever, we have a long, graduated
scale, in which many of the precious stones appear, those containing
iron in their composition being more or less responsive, as already
mentioned, and that either in their normal state, or when heated, and
always to an extent depending on the quantity or percentage of iron
they contain.
In
this case, also, science has not as yet been able to introduce into an
artificial stone the requisite quantity of iron to bring it the same
analytically as the gem it is supposed to represent, without completely
spoiling the colour. So that the behaviour of a stone in the presence
of a magnet, to the degree to which it should or should not respond, is
one of the important tests of a genuine stone.
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