124 C. H. Warren — Calcium Carbide, 



may show low colors while higher colors appear in thinner 

 portions. These anomalies are observed in grains from 

 a single rectangular cleavage piece and in ones which 

 show no indication of distortion incident to crushing 

 the sample ; and such distortion is easily deteced by the 

 bending and disturbance of the twinning line and lamellae. 



Extinction. — The extinction in calcium carbide is in 

 general parallel to the rectangular cleavages. For 

 the calcium carbide which has a yellowish color, and 

 contains probably some substance in solid solution, the 

 extinction, when definite, is in general inclined to rectan- 

 gular cleavages. The angle of inclination observed ranges 

 from a few degrees up to 22° or 24°. The common 

 values observed are from 12° to 22°. Rarely a yellowish 

 fragment has been observed with an approximately par- 

 allel extinction, but in the great majority of cases in 

 inclined extinction is associated with a yellow color. 



Nearly isotropic sections. — In preparations made 

 from the carbide, cleavage fragments may be frequently 

 found containing square or rectangular areas, charac- 

 terized by a series of rectangular markings (parallel 

 or perpendicular to the cleavages) on the surface, which 

 remain for the most part almost completely dark 

 (nearly isotropic) upon rotation of the grain between 

 crossed nicols in parallel light. These dark areas are 

 never quite homogeneous in appearance and are fre- 

 quently crossed or penetrated by lamellag which do not 

 remain dark. This lack of crystalline uniformity is 

 most clearly seen when the areas are tested with sensi- 

 tive plate. Two or even three of these dark areas 

 have been observed in a single fragment measuring 

 approximately 0.1 mm. by 0.07 mm. In fact they vary 

 considerably in size from very minute squares up to ones 

 at least 0.05 mm. on a side. It has been frequently 

 observed that these areas are surrounded by series of 

 lamellae parallel to the sides showing interference colors 

 which rise regularly in order in a direction away from the 

 area. 



The facts above set forth indicate that these dark areas 

 are of complex structure, like carbide in general, and that 

 the apparently isotropic character is brought about by a 

 crossing (through twinning) of a number of doubly 

 refracting lamella of such thickness, and such a manner, 

 that the individual plates have a compensating effect 



