Alumina with Silica. Lime and Magnesia. 325 



In every preparation of sillimanite examined, there was 

 present between the sillimanite fibers an isotropic substance of 

 much lower refractive index, about 1 # 530. This same sub- 

 tance appears in other preparations of the alumina-silica 

 series, and is probably glass, since its refractive index n, about 

 1*525-1*530, agrees with that of sillimanite glass obtained by 

 extremely rapid quenching of the melt from the iridium fur- 

 nace. Sillimanite crystallizes with great rapidity, but its 

 melting point is high and probably in the iridium furnace 

 preparations, where comparatively rapid cooling goes on 

 throughout the region of rapid crystallization for this silicate, 

 not enough time was given for the entire melt to crystallize 

 out, and small threads of glass are included between the crys- 

 tallized fibers and laths. If the refractive indices be used as a 

 criterion, crystallization in sillimanite means high molecular 

 condensation, and, as a result, numerous air spaces and cavities 

 appear in the crystallized mass. 



The presence of glass and minute elongated air cavities 

 tended to render the optical tests for homogeneity in prepara- 

 tions adjacent to the pure compound uncertain. No free 

 corundum was observed in the preparation Si0 2 35, A1 2 3 65, but 

 it was readily detected in the preparation Si0 2 30, A1 2 3 TO. 

 So far as the optical evidence goes, solid solution of A1 2 3 in 

 Al 2 Si0 5 may extend to the composition Si0 2 35, A1 2 3 65, but 

 not to SiO 2 30, Al 2 O 3 70. The refractive indices of the sil- 

 limanite fibers in the preparation Si0 2 35, A1 2 3 65, were practi- 

 cally identical with those of the pure compound, but measure- 

 ments of a high degree of accuracy were not possible, because 

 of the character of the material. 



Magnesium Oxide : Artificial periclase. — Preparations of 

 this oxide were made both by crystallizing the pure melt in 

 the electric arc and also from fluxes of magnesium chloride and 

 silica. The crystals from the latter were well developed and 

 octahedral in habit with occasional small cube faces. From 

 the melt they occur as rounded grains often irregular in shape 

 and without polyhedral outline in aggregated clusters and 

 masses. Cubic cleavage is well marked even on the grains and 

 was produced directly on the octahedral crystals. Octahedral 

 cleavage if present is not distinct. The crystals and grains are 

 colorless and perfectly isotropic with refractive index 1*734 ± 

 •002, determined by immersion method. The hardness is 

 about 6, apparently slightly above 6, since the grains appeared 

 to scratch adularia very slightly. Solid solution in periclase 

 is not great if it occurs at all. In a preparation MgO 90, CaO 

 10, free lime was present in the usual rounded grains, while 

 the refractive index of the periclase was practically unchanged. 



Am. Jour Sci.— Fourth Series, Vol. XXVIII, No. 166.— October, 1909. 



99 



