42 SCIENCE PROGRESS 



ordinary temperatures, the compounds in the glass may not be 

 those stable at high temperatures. 



The latter of these is overcome by A. L. Field and P. H. 

 Royster (U.S. Bureau of Mines, Tech. Paper 189, 191 8), who 

 have investigated the temperature-viscosity relations of part 

 of the system CaO — A1 2 3 — Si0 2 . The compositions of the 

 mixtures examined lay within the stability fields of calcium 

 metasilicate and gehlenite (Ca 2 Al 2 Si0 7 ). In a binary mixture 

 of the two silicates, the isothermal viscosity curves show a 

 minimum for the eutectic mixture, and a maximum nearer the 

 wollastonite end of the series, the discontinuities being much 

 sharper at lower temperatures (e.g. i35o°C.) than at high 

 temperatures (e.g. i6oo°C). In the ternary system, maxima 

 occur at the quintuple points, and minima at the binary eu- 

 tectics. Since the stability fields of the compounds are deter- 

 minable from viscosity measurements, it follows that the 

 compounds exist in the liquid state. Although this is appar- 

 ently in opposition to the view of R. B. Sosman (Trans. Fara- 

 day Soc. 12, 170, 1 91 6) that such compounds as diopside are 

 dissociated in the liquid state, the two cases are not strictly 

 analogous. Gehlenite, for example, is probably the calcium 

 salt of an aluminosilicic acid and is therefore analogous to such 

 a salt as potassium ferrocyanide, while diopside, on the other 

 hand, is probably a " molecular compound " and is analogous 

 to such double salts as potassium zinc sulphate. Further, the 

 system CaSi0 3 — MgSi0 3 is complicated by the fact that clino- 

 enstatite (MgSi0 3 ) breaks up below its melting-point to the 

 orthosilicate and free silica. 



Some further work on the interesting question of the 

 stability of the silica minerals has been carried out by J. B. 

 Ferguson and H. E. Merwin (Amer. Jour. Sci. 46, 417, 191 8 )» 

 who have redetermined the melting-points of tridymite and 

 cristobalite. The former is found to melt at 1670°^ io°C. 

 and the latter at i7io°± io° C. The authors also succeeded 

 in converting pure quartz to a mixture of tridymite and cristo- 

 balite by the action of dry heat at i35o°C, and tridymite 

 similarly to cristobalite at a temperature in the neighbourhood 

 of the melting-point of the former. This work affords some 

 verification of le Chatelier's contention (Bull. Soc. jr. Min. 40, 

 44, 191 7) that tridymite is stable above the temperature, 

 i470°C, given by C. N. Fenner (Amer. Jour. Sci. 36, 331, 1913) 



