11. PLASTICS. 



1. Vitrification, or Glass-making. 



We offer a few points in relation to glass, plain and colored, 

 and introduce, also, the subject of gems, as most allied to 

 glass. 



Bohemian Crlass. — The glass of which combustion tubes are 

 made has been examined by Ronney, and found to consist of : 



Silicic acid 73.13 



Lime 10.43 



Alumina 0.30 



Sesquioxide of iron 0.13 



Magnesia 0.26 



Protoxide of manganese 0.46 



Soda 3.07 



Potassa 11.49 



99.27 

 . Optical Cflass. — Maes and Cl^mandot (Comtes Rendus,1849), 

 having studied the influence of borax in the manufacture of 

 glass, have announced that the borosilicates of potassa, with 

 lime, soda, or zinc, are eminently suited for optical purposes, 

 owing to their remarkable hardness and transparency. 



Colo7^ed Crlass. — See an excellent essay by Bontemps, on the 

 substances used for colored glass, in the Phil. Mag. (3 ser.) 

 XXXV. 439. 



Aventurine Cflass. — Wbhler and others analyzed this glass, 

 which comes from Venice ; but Fremy and Clemandot have 

 lately imitated it. (Comptes Rendus, Fevrier, 1846.) They 

 heated a mixture of 300 pts. powdered crystal glass (glass 

 with a less portion of lead than flint-glass), 40 pts. suboxide 



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