C. H. Warren — Calcium Carbide. 123 



ticularly between crossed nicols, that the grains .are 

 entirely made up of groups of lamellae, which run either 

 parallel or perpendicular to, or at an angle of approxi- 

 mately 45°, to the cleavages. The lamellae often wedge 

 out parallel to their elongation, or may end against mem- 

 bers of another set. Different sets are commonly 

 arranged so as to form rectangular patterns. A single 

 plate examined after successive grindings showed differ- 

 ent sets of lamella3 for different degrees of thickness. 

 Changes of focus show the same. The greatest width 

 observed by the author for any single lamellae was 0.045 

 m. The larger groups of broader lamellae appeared 

 often to enclose rectangular areas of finer lamellae. 



There appears also to be a well-defined tendency in car- 

 bide to separate or part in a direction parallel to the 

 twinning plane. It has also been observed that the 

 twinning structures are as a rule more finely complex, 

 carbide having a yellow color. 



The whole structure is most complex. The observa- 

 tions made macroscopically and microscopically show 

 that the twinning and composition plane have a direction 

 approximately 45° to the cleavage direction, that is par- 

 allel to any one of the six diagonals of the cleavage angles. 

 With three rectangular directions of apparently almost 

 equal cleavage value and a twinning parallel to the diag- 

 onals, we have a crystalline structure that simulates 

 geometrically that of crystals of the isometric system, 

 viz., cubical cleavage and a dedocahedral twinning. Its 

 optical properties show that it is not isometric, but it 

 seems appropriate to describe it as geometrically 

 pseudo-cubic. 



Indices of refraction. — The indices of refraction as 

 determined by comparison with that of methylene 

 iodide (index 1.75) by the Becke & Schroeder v. der Kolk 

 method are always higher than 1.75. 



The double-refraction is strong and may be expressed 

 numerically as at least 0.050. 



In almost every fragment, with the possible exception 

 of very small ones, evidently portions of single lamellae, 

 the distribution of the interference colors furnishes, as 

 has been noted, proof of the polysynthetic character of 

 the grains. Single grains of uniform thickness may show 

 all variations of colors from a very dull gray to bright 

 colors of 2d and 3d orders. Thicker portions of grains 



