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LETTER II.
On the Effects of different Menſtruums
on Copper.
SIR,
IN a Letter of the 19th of laſt Month, which you did me the Honour to read before the Royal Society, I endeavoured, principally by means of ſome Experiments I had been lately making, to ſettle the Queſtion ſo much diſputed among the preſent Naturaliſts, Of, what the blue Gems in general are coloured from. What engaged me in the Diſpute, was an Objection raiſed againſt the Opinion I had declared myſelf of in this Caſe, in my Notes on Theophraſtus: And I am very happy to find, that even the Gentlemen who made that Objection are now convinced, that it is to Copper alone that the Sapphire and Turquoiſe owe their beautiful Blue.
For myſelf, I muſt acknowledge, that tho' I have long been convinced of the Fact, the Manner in which it was effected, was long a great Difficulty to me: The Menſtruum in which my Tincture of Copper, (which proved
