Cristobalite and Tridymite. 421 



previously been heated to approximately the desired tem- 

 perature. After the heat treatment, the tube was quickly 

 withdrawn and the charge plunged into mercury. When 

 cold the charge was removed from the thermoelement, 

 opened, and the sample examined microscopically. In 

 some experiments two charges instead of one were tied to 

 the thermoelement, but on account of the small diameter 

 of the furnace tube more could not be used at one time. 



Results. 



1. CaO-MgO-Si0 2 charges made up in the usual manner by 

 repeated grindings and heatings to about 1600°. Crystals of 

 cristobalite form during the heatings and are present in the 

 charge used for quenchings. 



(a) CaO 24, MgO 7, Si0 2 69. 



Temperature Sample Quenched 



(corrected). 



after : 



Results. 



1691° 



30 min. 



Almost entirely glass but a few 

 grains of cristobalite which 

 appear to be dissolving in the 

 glass. 



1675° 



120 min. 



Glass. 



Part of this latter quench 

 heated again. 



1638° 30 min. A number of good crystals of 



cristobalite. 

 A second part of this latter 

 quench taken and brought to 

 the required temperature in 6 

 minutes. 



1638° 20 min. A number of good crystals. 



(6) CaO 11-5, MgO 3-5, Si0 2 85. 



Temperature Sample Quenched 

 (corrected). after: Results. 



1718° - 30 min. Glass. 



1708° 30 min. A quantity of large cristobalite 



crystals and some glass. 



2. Silica glass tube heated for y 2 hour at 1550° by R. B. Sos- 

 man. Material : cristobalite with specks of more highly refract- 

 ing glass. 



