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it colour by the flame of wood it is kept too long in astrong heat, changes to an amethyst blue.

Page 114. Gold plates for enamelling.

That a certain mixture of alloy is necessary in thegold plates designed for being enamelled on, I have relatedon the authority of a writer in the French Encyclopedic-,who fays the gold must be of the fineness of twenty-twocarats at most, that if finer it will not have strengthenough, and that if coarser it will melt. I am told by anexperienced artist that this is a mistake; that ducat gold isgenerally used, whose fineness appears to be from 23s to23 ’- carats; and that the finest gold is for this use the best,unless where some parts of the gold are left bare and after-wards polilhed, as is often done in watch cafes, snuffboxes,.&c. for which purposes a mixture of alloy is necessary;that silver is preferred as an alloy to copper, the latter dis-posing the plates to tarnish and turn green; and that theplates are strengthened, by covering them on the backsides with enamel. Thus much is certain, that the finerthe gold, the more soft and flexible ; and the coarser, themore fusible it proves.

Page 121, 123. c Pouchstone-—not of the marble kind.

Mr. Pott also has taken notice, that the touchstone isnot of the marble kind; and that black marbles, how wellsoever they may answer for receiving a coloured strokefrom metals, are unfit for the use of touchstones, on ac-count of their being dissolved by aquafortis. He makesthe touchstone a clayey slate, partaking of iron; and finds,that in a strong fire, like many other ferrugineous clayeyminerals, it melts perfectly into a blackish brown flag, and

that.