XXXIV. On the crystallizations observed on glass.

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Abstract
The peculiar figure of rock-crystal has been long observed. Many other substances, as spars, precious stones, pyrites, ores, metals, salts, water, and oil, are also known to affect an uniformity of shape, when they are exposed to certain degrees of heat, cold, fluidity, and other necessary circumstances. From their resemblance in this respect to rock-crystal, they are said, when they assume their peculiar forms, to crystallize; and the regularly-shaped bodies, into which these substances concrete, are also called crystals. In many substances, when broken, the parts appear to have some determinate figure. This determination of figure, or grain, as it is called, is obvious in bismuth, regulus of antimony, zinc, and all other metallic bodies, which may be broken without extension of parts; and although the ductility of gold, silver, lead, and tin, prevents the appearance of the peculiar grains, when pieces of these metals are broken, yet we have reason to believe, that, by exposing them to proper circumstances, they also would shew a disposition to this species of crystallization, as it may be called, by a further extension of that term; for Mr. Homberg has observed, that when lead is broken while hot, in which state it is not ductile, a granulated texture appears.

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