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period cannot be considerable. Perhaps also this firstdegree of change may depend in part on an alterationproduced, by the heat, in die glass itself considered as acompound, or in the nature of its alkaline ingredient.The remarkable differences, in point of brittlenefs, whichhappen to glass merely from the quick or flow manner inwhich it is cooled, are well known : and as to alkalinesalts, when exposed for some time to a moderate fire,a part of them is always found to lose its saline nature,and become an earth.

IV. Of