Page:Popular Science Monthly Volume 83.djvu/30

26 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

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

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.