Alumina with Silica, Lime and Magnesia. 315 



5. Of these aluminates it seems probable that only 3CaO. 

 A1 Q 3 will occur in portland cement. 



6. There is one compound, MgO.Al 2 3 , between magnesia 

 and alumina. 



7. There is reason to believe that the system MgO-CaO is a 

 eutectic series with no compound and little if any solid solution. 

 The temperature range is too high for satisfactory investi- 

 gation. 



Having established the nature of the binary systems, experi- 

 mental study of the ternary system CaO-Al 2 3 -Si0 2 is now 

 under way. 



Optical Study ; by Fred. Eugene Wright. 



In the foregoing pages the general problem of the lime- 

 alumina series has been considered in detail, the different com- 

 pounds in the series have been described and their stability 

 relations at different temperatures discussed ;. in brief, all data, 

 chemical, thermal and optical, which bear on the problem, have 

 been used, and the conclusions reached are based on the entire 

 evidence at hand. In this treatment of the problem, its 

 physico-chemical aspects have been especially emphasized 

 because the established generalizations of physical chemistry 

 best explain and define the limits in the relations of the com- 

 bining minerals, and such data are of fundamental significance 

 in the general study of rock and ore genesis. A restatement 

 of the entire lime-alnmina problem from the optical standpoint 

 with the thermal and chemical data as confirmatory evidence, 

 although possible, is, therefore, deemed unnecessary, and in the 

 following pages only the detailed optical description of the 

 different components of this series will be given, followed by a 

 brief account of the character, significance and interrelation of 

 the different kinds of experimental evidence which require to 

 be brought to bear upon such problems of pedogenesis for 

 their effective solution. 



Calcium Oxide. — Crystals of this substance were not pre- 

 pared especially for this investigation, since its optic properties 

 have been described in detail in a former paper.* Free lime 

 crystallizes readily in the isometric system, is isotropic and 

 occurs in the different preparations of this series in the form 

 of rounded grains. A remeasurement of the refractive index 

 by the immersion method in a liquid consisting of methylene 

 iodide, arsenic bromide and arsenic sulphide, was made, and the 

 index found to be 1*832 ±'005. The high probable error is 

 partly due to the lack of distinctness of the phenomena 

 observed and to the slight attack of the lime bv the solution 

 itself. 



* This Journal (4), xxii, 294, 1906. 



