about one fiftieth Part. The Lead gives, in this Caſe, an additional Denſity to the Glaſs, which adds greatly to the Luſtre of the counterfeit Gem; as the more denſe the tranſparent Matter is, the more bright and vivid the metalline Tinge appears through it; but while Lead thus increaſes the Denſity, it debaſes the Glaſs in another reſpect of equal Conſequence, in that it makes it ſofter. Whichever of theſe Subſtances, however, is made the Subject of this Experiment, the Effect will be the ſame; for if we bring to the Trial of only a clear Charcoal Fire, a genuine Sapphire, and either of theſe factitious Subſtances, and throw them together into it, we ſhall ſoon ſee that they owe their Colours to Particles of a very different Kind; for the Genuine will be ſeen to emit a fine clear blue Flame, the Counterfeit not ſo much as the leaſt Vapour; and when, after this, they are taken out together, the true Sapphire ſhall be found wholly colourleſs and tranſparent as a Piece of Cryſtal, and the Counterfeit or Glaſs, unaltered.
Fire, which is thus able to diveſt the Sapphire of its Colour, has alſo the fame Effect on the Turguoiſe; as the Workers on it well know: And this is eaſily accounted for, if they are coloured, as I am convinced they are, by a fine metalline Sulphur. But I will venture to affirm, that it could not be the Caſe, if thoſe Gems were coloured by a Zaffer.
Let it not be here objected, that the Workers on the native Turquoiſes are obliged to have
Recourſe
