��THE POPULAR SCIEXCE MOXTIILY
��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 mctastable state, consequently it has the power of returning to
���Fig. 4. DEVixuiFitu Gi.as.s
��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 appear-
��Obsidian from which Cry.stals have nEOix to Separate. Magnified about 300 diameters.
��ance of ground glass. Its surface boconios rouirh and the glass is no longer ti'ansparont. Pieces of glass a]iparatus wliich have partially crys- tallized can he lound in any eliemiial hihoratory. It is called devitrified glass.