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THE POPULAR SCIENCE MONTHLY.
acetate in one respect only: its viscosity has reached the solid stage. Glass is an undercooled substance and like other undercooled substances is 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 which water has been allowed to pass, frequently shows this property when heated for a few minutes. The glass crystallizes, taking on the appearance
Fig. 5. Obsidian from which Crystals have begun to Separate. Magnified about 300 diameters.
of ground glass. Its surface becomes rough and the glass is no longer transparent. Pieces of glass apparatus which have partially crystallized can he found in any chemical laboratory. It is called devitrified glass.
