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CHAP. X.

ON THE FORMATION OF LENSES.

Preparation of the necessary Tools. - Choice of Glass. - Grinding. - Po.

lishing. — Curdled Lenses. - Means used for avoiding this Defect. 242

CHAP. XVI.

ON THE DEVITRIFICATION OF GLASS.

First observed by Neumann. — Experiments of Reaumur. Substance

known as Reaumur's Porcelain. Inappropriateness of this Name.

Uses to which the Substance may be applied. - Common Bottle Glass

most proper for this Conversion. — Method of effecting the Change.

Produced solely by Heat. — Experiment of Dr. Lewis. – Revitrification.

Experiments of Sir James Hall. - Proposal suggested thereby. - Observ.

ations of Guyton-Morveau. - Artificial Intaglios. - Mock Onyxes. -

Power of devitrified Glass to bear sudden Changes of Temperature. — Ex.

periments with coloured Glass. - Glass devitrified by burning Lava. — The

process promoted by multiplying the Ingredients of Glass. – - Devitrified

Glass conducts Heat more perfectly than when vitreous. - Becomes a

Conductor of Electricity. - Retains this Property when revitrified. 317

MANUFACTURE

OF

PORCELAIN.

B

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