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of the alkali is forced out, that there is scarcely enoughleft to make the earthy basis of the glass cohere. Thusglass, whose production has been commonly supposed tobe the utmost limits of the power of fire, has its earthand its salt, which one degree of fire had so firmly united,almost wholly disjoined by another.

If these principles be just, they afford a satisfactory ex-plication of the most remarkable phenomena of the pro-cess; as of those glasses only being convertible into por-celain which are prepared with saline matters, of thosebeing easiest changed which have least salt, of the samechanges being producible by cementation with very dissi-milar materials, of the glass becoming less and less fusi-ble in proportion to the change, of its becoming moreand more hard to a certain point, and afterwards more andmore brittle.

It has been observed above, that glass, in its conversioninto a fibrous porcelain, did not receive any increase of itsweight: it must be added also, that it scarcely sufferedany sensible diminution. I do not however apprehend,that this experiment is altogether irreconcilable withthose which seem to prove the exudation of the alkalinesalt of the glass. The strong marks of the exudation donot appear till after the porcelain has considerably passedits fibrous state ; and then the sand, concreting insepa-rably upon its surface, prevents our being able to deter-mine any thing from the weight. Green glass has assmall a proportion of salt, as can be made to vitrefy theearthy matter in the strongest sires of the glass-house :it is probable therefore that the separation of a very mi-nute portion of the salt may be sufficient for producingthe first degree of change, or rendering the mixt nolonger glass; the fusibility of the fibrous porcelain seemsto be a proof, that the quantity of alkali separated at this5 period