And sometimes holding the Stone upwards, I rubb'd its Broad side with
a fine smooth piece of Transparent Horn, by which means the Light through
that Diaphanous Substance, did whilst I was actually rubbing the Stone,
appear so Brisk that sometimes and in some places it seem'd to have little
Sparks of fire.
Eighteenthly, I took also a piece of flat Blew Glass, and having rubb'd the
Diamond well upon a Cloath, and nimbly clapt the Glass upon it, to try
whether in case the Light could peirce it, it would by appearing Green, or
of some other Colour than Blew, assist me to guess whether it self were
sincere or no. But finding the Glass impervious to so faint a Light, I then
thought it fit to try whether that hard Bodies would not by Attrition
increase the Diamonds Light so as to become penetrable thereby, and
accordingly when I rubb'd the Glass briskly upon the Stone, I found the
Light to be Conspicuous enough, and somewhat Dy'd in its passage, but found
it not easie to give a Name to the Colour it exhibited.
Lastly, To comply with the Suspition I had upon the whole Matter, that the
chief manifest Change wrought in the Stone, was by Compression of its
parts, rather than Incalescence, I took a piece of white Tile well Glaz'd,
and if I press'd the Stone hard against it, it seem'd though I did not rub
it to and fro, to shine at the Sides: And however it did both very
manifestly and vigorously Shine, if whilst I so press'd it, I mov'd it any
way upon the Surface of the Tile, though I did not make it draw a Line of
above a quarter of an Inch long, or thereabouts.
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