DEVELOPMENTS IN GLASS INDUSTRY 143 



cause: this body functions as an acid, setting free silica. 

 As has already been pointed out, the physical and chem- 

 ical investigations at the Jena Glass Works were in the first 



. Upper limiting temperature of devitrification 



_____ Curve of equal tenacity 

 FIG. 1 



place aimed chiefly for the production of new optical 

 glasses. All tHe glasses known in the earlier days were 



