Rankin and Wright — Ternary System CaO-Al 2 0-Si0 2 . 3 



have clearly before us the conclusions upon which the present 

 work is based and partly because improved methods of melting 

 point determinations have resulted in the establishment of new 

 temperature values for some of the points. 



The Three Components : Lime, Alumina, Silica. 



Lime, CaO. The melting point of pure CaO has recently 

 been determined by Kanolt* as 2570° ; Kanolt used a graphite 

 furnace and took extreme precautions to prevent contamination 

 of the charge. It is difficult to state just how accurate this 

 point may be considered to be, but it is the best available at 

 the present time. No indications of a second form have so far 

 been observed. 



Well-developed crystals of lime were obtained only from 

 relatively large melts of pure CaO. The crystals are colorless 

 or pale yellow, transparent, and were observed only in 

 unmodified cubes of the isometric system. Cleavage perfect 

 after the cube (100). The crystals are isotropic ; occasionally 

 the larger individuals show optical anomalies, due probably to 

 internal strain. In the preparations the free lime appears 

 usually in tine isotropic grains, usually rounded in outline and 

 of very high refractive index (about 1*83). The grains are 

 easily recognizable, even when only a few thousandths of a 

 millimeter in diameter. 



Alumina,\ Alfi z . Artificial Corundum. — The melting 

 point of A1 2 3 as determined by KanoltJ is 2050°. 



Optically, the properties of artificial corundum agree well 

 with those of the natural mineral. Grains crystallized from a 

 melt of pure Al a O, rarely show crystal outline ; they are 

 rounded in outline and less than 0*05 Inm in diameter. Minute 

 air cavities in the grains are abundant and characteristic. 

 Hardness 9. Refractive indices: o> Na = 1*768 ± 0'003 ; e Na = 

 1-760 ±0-003; birefringence fairly weak, about 0*009. Uni- 

 axial, optically negative. From mixtures it usually crystallizes 

 in thin, sharply defined hexagonal plates. 



Silica, SiO^ An extensive investigation of the stability 

 relations of the different crystal modifications of Si0 2 has been 

 carried out in this laboratory by C. N. Fenner.§ Briefly sum- 

 marized, the results of this investigation are : 



* J. Wash. Acad. Sci., iii, 315, 1913. 



t A second form of A1 2 3 has recently been observed during work on the 

 ternary system CaO-Al 2 3 -MgO. A complete description of this new form 

 will be given when that system is published. 



% J. Wash. Acad. Sci., iii, 315, 1913. 



§ This Journal (4), xxxvi, 331, 1913. 



