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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
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