An essay of the great effects of even languid and unheeded motion whereunto is annexed An experimental discourse of some little observed causes of the insalubrity and salubrity of the air and its effects / by the Honourable Robert Boyle ...

To the IXth. Chapter. Pag. 110.

Observation I.

BEcause there are divers Gems; particularly those Transparent ones that are Red or Blew, that are much harder than Iron or steel, it may much strengthen the Proof of our 8th Observation, if I here relate that a Jeweller to a great Princess an∣swer'd me, that when he polish'd Sa∣phyrs, Rubies, and some sorts of o∣ther hard Gems, upon his Mill, they would seem when attrition had made them very hot, to be all on Fire, like so many little Coals: And that each of them had the light it afforded ting'd with a Colour proper to the Stone; so that the Ruby gave a Red Light, the Saphyr a Blew, &c. And I remember that inquiring of a skilfull Cutter of Diamonds and Polisher of Gems, whose Customer I had been, about some Conjectures I had concerning things belonging to his Profession, he

answer'd me that sometimes, when he polish'd certain Stones, especially Rubies, that were pretty large and perhaps not thick, he could plainly perceive that the Stone gap'd at and near the Edge, as if it were begun to be crack'd; which sign admonish∣ed him to make haste to slacken the Motion of the Mill, lest the Stone should absolutely burst; which if it did not he could not perceive any Flaw in it when it was throughly cold, but, which was strange, it appear'd as entire as ever. He added, in con∣firmation of what he had said of the intense Heat that Gems would some∣times acquire by Attrition, whilst they were in polishing, that having lately given by this means too great a degree of Heat, to an Oriental Topaz (which sort of that Gem is very hard,) it crack'd upon the Mill, in so much that one part of it quite separated from the rest, and spoil'd the Stone in the capacity of a Gem; as a Proof where∣of he had laid it aside for me, and would needs make me accept it, as

a curious, though not an usefull, thing.