Dihexagonal Alternating Type. 213 



crystal along the cleavage lines, which rapidly developed 

 into deep canals, before definite figures could be seen. It 

 is readily noticed, however, that a change in the concen- 

 tration of the acid produces a very apparent change in the 

 shape and position of the figures, but none of them 

 reveals any planes of symmetry. 



The third solvent used was HN0 3 . This was prepared 

 by taking a few c.c. of cone. C.P. HN0 3 and diluting it to 

 ten times its volume with water. This was also the 

 strength of the HC1 used for the purpose of etching this 

 form. Twenty seconds' immersion produced well-formed 

 figures, very similar to the HC1 forms (see fig 9). They 

 are irregular, rhombic in shape, slightly elongated par- 

 allel to the c axis. A few of the figures are bounded by 

 parallel faces, but the most of them possessed but a single 

 pair of faces lying in parallel position, the larger faces 

 often being slightly divergent. Occasionally several fig- 

 ures are intergrown producing a large irregular four- 

 sided pit, very often containing small figures. The 

 appendages occur on a few figures, but they are very 

 rare, as the longer sides of the figures are quite straight 

 and not curved as are the HC1 forms. The canals run- 

 ning across the face bear the same relation to the etch 

 figures as do those produced by HC1, the more regular 

 depressions extending parallel to the cleavage. The fig- 

 ures possess no plane of symmetry and indicate by their 

 positions on adjacent faces an alternating axis of sym- 

 metry c. 



Thus the great similarity of the HN0 3 and HC1 figures 

 is again shown in the etchings occurring upon the second- 

 order prism; the figures produced by citric acid are 

 essentially different, due to the difference in the acids, but 

 there is a general likeness to be observed among the etch- 

 ings of all acids used, especially when compared to the 

 etch figures produced upon the unit prism 1010 by the 

 same solvents. As one would expect, there is a tendency 

 toward a greater similarity in the figures produced upon 

 the same face by different solvents than upon different 

 faces by the same solvent. 



Scalenohedron — The scalenohedron, 2131, could not be 

 etched successfully with any of the common laboratory 

 acids, although different concentrations and tempera- 

 tures were tried. It was very apparent, however, that the 

 face had been attacked, but the action had been more or 



