26 



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- 



FiG. 5. 



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. 



