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glass into porcelain, melted and united with it into onelemivitreous lump.

I tried likewise colcothar, or the red calx of iron,which remains from vitriol after the acid has been expell-ed by fire. Pieces of green glass being surrounded withthis powder, and baked for several hours in the upperchamber of the wind-furnace, the glass and colcotharwere all found to have run together into a black mass,externally rough, internally somewhat smooth and caver-nulous, of considerable hardness so as to strike fire freelywith steel. It is pretty remarkable, that a metallic sub-stance so refractory in the fire, should be so greatly disposedto melt with green glass.

SECT. IV.

Experiments of the baking of different forts of Glass, and ofbodies approaching to a •vitreous nature.

L A S S consists of earthy or stony substances, ory metallic calces, brought into fusion and transpa-rency in a strong sire. Pure unmixed earths cannot bemade to vitrefy by any known degree of fire ; but fre-quently one kind of earth is made vitrescible by mixingwith it a certain proportion of a different one, which sepa-rately is as unfusible as the other : thus clay and chalk,though each by itself is altogether unfusible, yet whenmixed together in due proportions, melt and form a trulyvitreous compound : in several of the experiments I havebeen giving an account of, the crucibles were found partlyvitrefied, not on the outside which had been immediatelyexposed to the fire, but on the inside, which had been incontact with earths of a different kind from those ofwhich the crucible was composed. The several sorts ofglass in common use are prepared however on another

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