Alumina with Silica, Lime and Magnesia. 321 



yellow of the first order. The optic axial angle was measured 

 by the double screw micrometer ocular 2 V = 35° ±5°. This 

 value is the average of seven different measurements and part 

 of the large probable error is due to the strong axial dispersion, 

 which is pronounced with 2Y p > 2V V . The attempt was 

 made to determine the angular amount of this dispersion, but 

 the axial figures observed were not sharp enough for precise 

 work and only the general statement can be made that 

 2V p is several degrees larger at least than 2E V . The optical 

 character is negative. The grains extinguish parallel with the 

 prismatic cleavage. The ellipsoidal axis a, and with it the 

 plane of the optic axis, is parallel with the direction of pris- 

 matic elongation of the crystals. 



The alteration to the optically positive form is clearly marked 

 in the powder. It proceeds from the surface of the grains and 

 works toward the center along the cleavage cracks and occa- 

 sional transverse cracks, so that as it proceeds the original 

 substance is replaced . by a fine mesh work of the optically 

 positive form. The alteration is accompanied by a slight 

 expansion in volume, about 2 per cent judging from the refrac- 

 tive indices, and this in turn tends to facilitate further change 

 by producing further cleavage cracks and causing the crystal 

 to break down entirely. The resulting product of change 

 resembles y-orthosilicate in appearance and this fact suggested 

 the existence of this probably unstable phase of the 

 3Ca0.5Al a 3 compound. 



The above optical data indicate that this compound is in all 

 probability orthorhombic in symmetry. Its chief character- 

 istics are refractive indices about 1*67, medium birefringence, 

 small negative optic axial angle with strong axial dispersion, 

 the plane of the optic axes and the ellipsoidal axis a lying 

 parallel with the prismatic cleavage direction of elongation. 



Aluminum oxide : Artificial corundum. — This substance 

 melts at an exceedingly high temperature, and in the present 

 series of experiments no special attempt was made to procure 

 measurable crystals. The optic properties were determined on 

 a fine-grained preparation prepared by heating fine impalpable 

 precipitated alumina in the iridium furnace to about 2100°. 

 The crystal grains thus formed are less than *05 mm in diameter 

 and of rounded outline. No definite crystal outlines nor 

 cleavage cracks were observed. Air cavities and minute 

 bubbles are characteristic and abundant. The hardness is 9. 

 The refractive indices were determined by the immersion 

 method, co = 1-768 ± -003, e = 1-760 ± '003. The bire- 

 fringence was found to be roughly *009 on a small section 

 parallel with the principal axis. 



