26 



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 appear- 



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mm 





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Fig. 5. Obsidian from which Crystals have begun to Separate. 

 Magnified about 300 diameters. 



ance of ground glass. Its surface becomes rough and the glass is no 

 longer transparent. Pieces of glass apparatus which have partially crys- 

 tallized can be found in any chemical laboratory. It is called devitrified 

 glass: 



