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of the ware softened, so as that the powder it was- sur-rounded with partly adhered, but in other respects therewas no sensible alteration. I tried likewise some of themore simple transparent and semitransparent stones, ascrystal and flint; which, by long cementations with dif-ferent materials, received no other change than the dimi-nution of hardness and transparency which simple heatproduces-in them.

II. Metallic Glasses.

Pure glass of lead, surrounded with sand, and bakedfor many hours in a moderate red heat, suffered no per-ceptible alteration, except where some of it had meltedoff and dissolved a part of the sand. Common flint glass,in which the flint or sand that makes its basis is vitrefiedchiefly by an addition of calx of lead, proved also unsub-duable by cementation : it became rough and brownishon the outside, and internally somewhat cloudy, but gainednothing of the appearance of porcelain by long continuedand repeated bakings.

I cemented likewise some glasses tinged with metallicbodies, as the common blue glass tinged with the prepa-ration of cobalt called zaffre, blue and green glasses wkhcopper, and the ruby glass already mentioned in the tenthsection of the history of gold. All of them retained theirvitreous appearance, and suffered very little change evenin their colour: the ruby glass grew somewhat darker,and one of the copper glasses more dull, but the blue glasswith zaffre did not appear to have received any alteration.The bone afh.es, with which all these glasses were sur-rounded, adhered to them pretty firmly, probably fromthe surface having been softened or partly melted by- theheat : on the zaffre glass and the ruby glass, the earthycrust remained white as at first : on all the copper glasses

it