ii8 THE NEW WORLD 'OF SCIENCE 



a great many interdependent factors. The time of starting and 

 stopping, the temperatures to be held throughout the operation, 

 and the path followed in the stirring are all important. The 

 stirrer must come near enough and yet must not be too close 

 'to either side wall or the bottom of the pot. 



Simple inspection methods for purposes of rough sorting 

 were evolved, and then more refined methods for use at other 

 points. An immersion method, using liquids of the same re- 

 fractive index as the glass, served well for locating striae in 

 rough glass chunks, and a combination of this method, the work 

 of the Bureau of Standards, with monochromatic light served 

 to detect the finest striae. 



We have referred to annealing and the progress made 

 in that art. The German practice has been to place chunks 

 of glass in square molds, and heat them until the glass would 

 flow into the shape of a plate, in which condition annealing took 

 place. The fall in temperature from 465 C. to 370 C. was 

 spread over an interval of four or more weeks. The American 

 practice is to heat the glass in a muffle to the so f ting point and 

 then to press it into the desired shape for grinding and polish- 

 ing. Annealing of these small pieces may then be done in 

 some instances in three days. 



As has been pointed out, there is a certain danger in the use 

 of the pot arch as a chamber in which to allow pots of glass 

 to cool. This danger comes from the re-melting of the stiff 

 crust and the skin on the sides of the pot, which form before 

 the pot is placed in the arch. This re-melting starts convec- 

 tion currents which may sweep into the glass, producing striae. 

 At a time when there was a scarcity of pot arches, the Pitts- 

 burgh Plate Glass Company tried banking the pots with sand 

 and this experiment led to the use of refractory lined iron 

 drums which might be let down over the pot. This practice 

 has been widely followed and is quite successful. 



This same company also worked out the rolling of optical 

 glass into sheets on a casting table, employing technique similar 

 to that in the manufacture of plate glass. The spectacle 



