I now return the smallest of your two tourmalins, with hearty thanks
for your kind present of the other, which, though I value highly for
its rare and wonderful properties, I shall ever esteem it more for
the friendship I am honoured with by the giver.
I hear that the negative electricity of one side of the tourmalin,
when heated, is absolutely denied (and all that has been related of
it ascribed to prejudice in favour of a system) by some ingenious
gentlemen abroad, who profess to have made the experiments on the
stone with care and exactness. The experiments have succeeded
differently with me; yet I would not call the accuracy of those
gentlemen in question. Possibly the tourmalins they have tried were
not properly cut; so that the positive and negative powers were
obliquely placed, or in some manner whereby their effects were
confused, or the negative parts more easily supplied by the positive.
Perhaps the lapidaries who have hitherto cut these stones, had no
regard to the situation of the two powers, but chose to make the
faces of the stone where they could obtain the greatest breadth,
or some other advantage in the form. If any of these stones, in
their natural state, can be procured here, I think it would be
right to endeavour finding, before they are cut, the two sides that
contain the opposite powers, and make the faces there. Possibly, in
that case, the effects might be stronger, and more distinct; for
though both these stones that I have examined have evidently the
two properties, yet, without the full heat given by boiling water,
they are somewhat confused; the virtue seems strongest towards one
end of the face; and in the middle, or near the other end, scarce
discernible; and the negative, I think, always weaker than the
positive.
I have had the large one new cut, so as to make both sides alike,
and find the change of form has made no change of power, but the
properties of each side remain the same as I found them before. It
is now set in a ring in such a manner as to turn on an axis, that I
may conveniently, in making experiments, come at both side of the
stone. The little rim of gold it is set in, has made no alteration in
its effects. The warmth of my finger, when I wear it, is sufficient
to give it some degree of electricity, so that it is always ready to
attract light bodies.
The following experiments have satisfied me that M. Æpinus's account
of the positive and negative states of the opposite sides of the
heated tourmalin is well founded.