. Popular science monthly. Fig. 2. Crystals Grow-ing ix a Tube of Under-cooled Sodium AcetateSolution. Fig. 3. 26 THE POPULAR SCIENCE MONTHLY acetate in one respect only: its viscosity has reached the solid is an undercooled substance and like other undercooled substancesis in the metastable state, consequently it has the power of returning to. Fig. 4. Devitrified Glass. the stable condition. Old glass, especially glass tubes through whichwater has been allowed to pass, frequently shows this property whenheated for a few minutes. The glass crystallizes, taking on the appear- . ——:—
. Popular science monthly. Fig. 2. Crystals Grow-ing ix a Tube of Under-cooled Sodium AcetateSolution. Fig. 3. 26 THE POPULAR SCIENCE MONTHLY acetate in one respect only: its viscosity has reached the solid is an undercooled substance and like other undercooled substancesis in the metastable state, consequently it has the power of returning to. Fig. 4. Devitrified Glass. the stable condition. Old glass, especially glass tubes through whichwater has been allowed to pass, frequently shows this property whenheated for a few minutes. The glass crystallizes, taking on the appear- . ——:— . • ? * «r- * v ?.••. ••,/ -V ,* ?: v £.:: * J * mm Sjl^fcfc^Ah • V ? * ? ,•- v >.V ?, ?• • ^ * . * m • * s - . faj•???:, ?•?! -r%* . . ><* P I- . •» »v ** $ . v -• ? * * Fig. 5. Obsidian from which Crystals have begun to about 300 diameters. ance of ground glass. Its surface becomes rough and the glass is nolonger transparent. Pieces of glass apparatus which have partially crys-tallized can be found in any chemical laboratory. It is called devitrifiedglass: SUSPENDED CHANGES IN NATURE 27 Certain glassy appearing minerals like obsidian are really under-cooled substances, natural glasses, which have cooled without crystal-lizing. In the course of time th
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