Alumina with Silica, Lime and Magnesia. 317 



distinct cleavage was observed, either in the crystalline powder 

 or in the thin sections of the different crystallized melts. In 

 the thin section, however, minute inclusions, apparently air 

 cavities, are often arranged in systems of parallel lines inter- 

 secting at different angles and occasionally there appears a 

 tendency to fracture along these lines, although it is not suf- 

 ficiently pronounced to be called even poor cleavage. The 

 sections are completely isotropic in all positions and the 

 crystal system is therefore isometric. The melts are frequently 

 colored especially in shades of yellow or brown, the color 

 being probably due to contamination with platinum from the 

 crucibles in which the melts were made. The luster is vitreous 

 and the fracture conchoidal and often interrupted. The hard- 

 ness is about 5. The refractive index, measured on the total 

 refractometer, was found to be ^ a =l*608±'002. The refrac- 

 tive indices of crystals from preparations slightly different in 

 composition from the 5Ca0.3Al 2 3 compound were measured 

 with the following results : 



CaO 43, A1 2 3 51 ?i NsL = 1-61 ± 1 "003 



CaO 49, A1 2 3 51 ?z Na = 1*61 1 ± "003 



CaO 51, Al 2 3 49 ?i Na = 1-611 ±'003 



Xone of these measurements were good, and in view of the 

 lack of homogeneity of preparations adjacent to the 5Ca0.3 A1 2 3 

 compound in chemical composition, it is evident that solid 

 solution in this compound is not pronounced, but is, in fact, 

 extremely limited if present at all. 



The refractive index of the glass of the composition of this 

 compound is about 1*662 ; it is interesting to note that in this 

 compound crystallization means molecular rarefaction and not 

 molecular condensation, which is usually the case. 



The unstable 5CaO.SAlfi % compound : CaO VT'7 8, Alfi, 

 5%'22. — The crystallographic development is much less favor- 

 able for optical examination than that of other members of the 

 lime-alumina series. The very fact of its unstable character 

 precludes crystal growth for a long period of time under any 

 but very special conditions, and as a result the material avail- 

 able for investigation is finely crystalline and usually intricately 

 intergrown, either as radial spherulites or in aggregates of over- 

 lapping and often roughly parallel fibers. The optic properties 

 which can be obtained with such material are few and less 

 accurate than those from well-developed crystals. The crystal 

 habit is fibrous to prismatic; cleavage if present is parallel 

 with fiber direction but not perfect. The luster is vitreous and 

 the color usually green and due possibly to slight admixture of 

 platinum from the crucible. The hardness is about 5. The 

 refractive indices were measured by the immersion method and 

 found to be a = l-687±"002, 7 = l'692±-002. The birefringence 



