Ferguson and Merwin — The Ternary System. 121 



2. The charge which contained only tridymite was 

 mixed with some of the original material forming a 

 charge containing both cristobalite and tridymite. 

 This material was treated as follows : 



(a) Heated at 1515° for 3 honrs. Both tridymite 

 and cristobalite identified. 



(b) Heated at 1485° for 3% hours. Both tri- 

 dymite and cristobalite present. 



(c) Heated for 16 honrs, the temperature falling 

 from 1500° C to 1^83 °C. Little if any change. 



3. A charge containing cristobalite + glass — heated 

 all night at 1464° C. No tridymite formed. 



These results confirm the observations of both Fenner 

 and Bowen 51 upon the sluggishness of this inversion, and 

 because of this sluggishness it is impossible to fix upon 

 an exact value for the inversion temperature in this 

 system. Our results certainly indicate a value in this 

 silicate system which lies below 1500° C with cristobalite 

 as the stable high-temperature form. Fenner found in 

 the tungstate melts, 1470 ± 10° C. Only slight, if any, 

 solid solution could have been present to affect our meas- 

 urements or Fenner 's, which are in substantial agree- 

 ment. 



The Binary Systems within the Ternary System. 



The presence within the ternary system of so many 

 compounds which melt incongruently and also so many 

 solid solutions, renders the binary svstems rather few 

 in number. The systems CaO.MgO.2SiO,— 2CaO.MgO, 

 2Si0 2 and 2CaO.Mg0.2Si0 2 — 2CaO.Si0 2 are of the 

 simplest type and are given diagrammaticaliy in figures 

 15 and 16 which were obtained by interpolation from the 

 melting temperatures determined for compositions lying 

 in the fields of these compounds. The system CaO. 

 Si0 2 — 2CaO.Mg0.2Si0 2 is more complicated including as 

 it does several series of solid solutions. The evidence 

 upon which the purely solidus relations are based will be 

 given in a later paper but for the sake of completeness the 

 general results are included in the diagram given in 

 %. 17. 



51 This Journal (4), 38, 245, 1914. 



