126 C. H. Warren — Calcium Carbide. 



ingly heavier than the other. On rotation of the frag- 

 ment, the cross appears to rotate somewhat abont^ its 

 o\\ai axis, splits open into hyperbolae, as with normal biax- 

 ial figures, but fades rapidly to white as the 45° position 

 is approached. Various gradations between these 

 last described figures and those which so far as can be 

 told are normally biaxial have been observed. 



On account of the fact that the yellow carbide very 

 rarety yields fragments exhibiting a uniform optical 

 structure throughout a sufficiently large portion of the 

 grain to permit of satisfactory study with convergent 

 light, it has not been found possible to establish the 

 position of the principal optical elements. Pojptions of 

 interference figures have, however, been obtained 

 which showed in white light highly colored curves lying 

 unsymmetrically in the field (the fragment lying on one 

 cle-avage face) and upon rotation a dark brush moving 

 across the field, the direction of its movement being oppo- 

 site to that of the rotation of the fragment. This indi- 

 cates that, as would be expected from the oblique extinc- 

 tion of the grains, the position of the optical elements is 

 unsymmetrical to the cleavages. , 



System of Crystallisation. — The double-refraction, the 

 rectangular cleavage and parallel extinction of calcium 

 carbide lead to the •conclusion that the system is either 

 tetragonal or orthorhombic. There was a difference of 

 opinion among the various observers as to which one of 

 the two systems the carbide really belonged to. The curi- 

 ously anomalous behavior of the interference figures is 

 puzzling and difficult of explanation, but in general it 

 might be produced by a complex twinning, be the system 

 of crystallization either tetragonal or orthorhombic. The 

 writer has always been, and still is of the opinion, thai 

 without definite proof, it is more in keeping with crystal- 

 lographic philosophy to consider the carbide as of ortho- 

 rhombic symmetry, with a polysynthetic twinning parallel 

 to the diagonals approximately at 45° to the pinacoids: 

 pseudo-duodecahedral). This twinning is mimetic, caus- 

 ing the carbide to appear pseudo-cubic geometrically and 

 pseudo-tetragonal optically. 



The yellow and less pure carbide, which, as has been 

 indicated earlier, is probably a solid solution with other 

 substances, is apparently triclinic. Geometrically (viz. 

 in cleavage) it appears not to differ noticeably from the 

 purplish or purer carbide. 



