426 Cristobalite and Tridymite. 



The thermoelements when calibrated at the Pd point 

 gave 16080 microvolts. A standard element gives 16140 

 microvolts. 



Discussion of Results on Tridymite and Cristobalite. 



This is, we believe, the first time that the inversion of 

 quartz to tridymite by dry heat has ever been observed 

 experimentally. In No. 2 we have inverted quartz to 

 tridymite and then this tridymite to cristobalite by means 

 of this agency alone. Tridymite melts sharply at a tem- 

 perature of 1670° ±10°. 



Our results confirm the earlier observations of Fen- 

 ner, 13 who considered the region of stability of cristo- 

 balite to be above that of tridymite. 



Summary. 



1. A new type of furnace has been described capable 

 of maintaining, in an oxidizing atmosphere, a charge at 

 temperatures slightly above 1700° for periods of time of 

 at least a few hours. This furnace is constructed on the 

 cascade principle ; the inner coil is of an alloy of plat- 

 inum with 20% rhodium, the outer coil of pure platinum. 

 The two coils are insulated from each other by well 

 burned magnesia powder and the inner coil is wound on a 

 helically grooved magnesia tube. 



2. The melting point of cristobalite has been redeter- 

 mined with the aid of the above-described furnace. The 

 new value is 1710° ± 10°C. 



3. The melting point of tridvmite has been determined 

 for the first time. It melts at 1670° ± 10° C. 



4. Quartz has been directly inverted to tridymite by 

 means of dry heat alone. 



The region of stability for cristobalite lies above that 

 for tridymite. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C. 



13 C. N. Fenner, this Journal, 36, 381, 1913. 



