Cosmo Johns — Allotropic Forms of Silica. 119 



He was also desirous of clearing up, if possible, the difficulty 

 caused by the conflicting values given to the fusion-point of silica 

 by different investigators. Joly,^ using the meldometer, found 

 quartz to fuse at 1200° C, and, with prolonged heating, even at 

 much lower temperatures. Boudouard,^ in the course of his 

 observations on calcic and alumino - calcic silicates, used Soger 

 cones and determined the fusion-point of silica to be 1830° C. 

 Other experimenters have given values varying from 1450° to 

 1550° 0. It is very evident that the differences are too great to 

 be explained by suggesting experimental errors as the cause. 



Advantage was taken of the fact that a regenerative gas furnace, 

 whose temperature could be regulated and only limited theoretically 

 by the dissociation point of carbon dioxide, was available to make 

 a series of experiments with quartz sand of a high degree of purity. 



After its specific gravity had been determined, a quantity was 

 introduced into the furnace, rapidly heated to 1500° C, and 

 maintained at that temperature for 48 hours. When withdrawn 

 the material was quite pulverent, and did not show the slightest 

 signs of fusion. It had, however, increased in volume and become 

 milk-white in appearance. Selecting the hottest part of the furnace, 

 where the temperature was determined to be above 1800° C, 

 another portion of similar sand was introduced and withdrawn after 

 30 minutes exposure to that high temperature. On examination 

 the surface of the mass was found to be fused to a depth of 

 7 or 8 mm., but the remainder was in the same pulverent state 

 as that found after the first experiment. 



Both experiments Avere repeated, but it was found that quartz 

 sand could only be fused with great difficulty, and only by selecting the 

 hottest part of the furnace. Yet at the same time the temperature 

 of the furnace was such that steel was being maintained in a fluid 

 state with ease. It would seem that Boudouard's determination is 

 confirmed, and the fusion-point of silica lies above 1800° C. 



Eeference has been made to the fact that the unfused sand had 

 increased in volume during both experiments. This was confirmed 

 when the specific gravity was determined, for the results were as 

 follows : — 



Original quartz sand 2*645 sp. gr. 



After prolonged exposure to 1500° C 2-309 ,, 



The specific gravity of the original quartz sand is that of a very 

 pure quartz, and this only confirms the analysis, which showed it to 

 be practically pure silica. The specific gravity of the altered and 

 unfused mass is the average of tridymite. The quartz had, in fact, 

 been transformed into another phase, and later experiments indicated 

 the transformation point to be about 1300° C. It is hoped to make 

 further experiments and determine the point more accurately. 



It becomes apparent that silica may exist in two solid phases, 

 viz. quartz and tridymite. Also that the former is only stable 



1 Rep. Brit. Assoc, 1900, p. 730. 



2 Journ. I. and S. Inst., vol. i (1905), p. 350. 



