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Common window glass appeared to suffer the samechange as the glass bottles, becoming opake, and inter-nally both fibrous and granulated according to the con-tinuance of the fire : but it was greatly disposed to blister,and part os the glass generally melted out.

The finer fort of glass called crown glass, and looking-glass plates, did not become porcelain at all. In a mode-rate heat they grew wrinkled and shrivelled on the surface,and less transparent than before, but still remained glass :on repeating the cementation with a stronger heat, theypartly melted and mixed with the surrounding powder,but did not appear to have suffered any other alteration.A glass which I had prepared myself from calcined flintand pure fixt alkaline salt, remained also unchanged, inthe same heat, and surrounded with the same powders,by which common green bottles had been turned into per-fect porcelain. Green glass itself, melted with an addi-tional quantity of salt, amounting to about a ninth part ofits weight, and then cemented with sand, continued like-wise unchanged.

The foundation of these remarkable differences may bepresumed, from this last experiment, to depend chiefly onthe different quantities of salt in the several sorts of glass.The vitrification of sand with wood-afhes is influenced nota little by the action of the two earths on one another;so that, though vehement fire reduces the mixture intoglass, yet the quantity of saline matter in the ashes ismuch less than would be sufficient for the vitrification ofthe two earths separately. In the other coarse sorts ofglass, a larger proportion of the ashes, and consequently ofsaline matter, is used, or some alkaline salt itself is added,to render the mass more fusible. In the fine glasses, thequantity of salt is still larger, the vitrification being effectedalmost wholly by this ingredient.

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