206 lioness — Etching Figures of the 



As a further comparison, the unit prism was etched 

 with 10 percent HN() :! , which acted upon the crystal in a 

 manner very similar to HC1. After immersion for 15 sec- 

 onds in the acid good figures were obtained (see fig 2) ; 

 several distinctions, however, are to be made. In the first 

 place, the basal plane, which is so prominent in the HC1 

 figures, is much reduced in size; it does not extend com- 

 pletely to the pit of the figure, but the two convergent 

 lines bounding it meet to form the apex at the center of 

 the figure and continue to the opposite end as a groove, 

 coincident with the intersection of the two lateral faces of 

 the figure. The apex angle as measured before varies 

 from 12° to 14°. This is much smaller than the angle of 

 the HC1 figure, which measured approximately 25°. This 

 face, still triangular, is reduced to such an extent by the 

 development of the two lateral faces, that one may call it 

 the fourth lateral face lying opposite the deeper end of 

 the figure. Its intersection with the crystal face is a 

 straight line more distinct than that observed on the HC1 

 etchings. The figures are elongated parallel to c with the 

 lateral boundaries curved outward, a distinction from the 

 HC1 fig-ures, which are slightly curved inward or straight. 

 Although the ends of the figures are alike in width, the 

 two faces occupying this position are very different ; the 

 one, four-sided, w T ith a slight vertical groove extending 

 downward, descends very abruptly to the deepest part of 

 the figure ; the other is an elongated triangle, described 

 above, which descends very gradually to the bottom of 

 the pit. The two lateral faces are decidedly enlarged as 

 compared to the HC1 figures, and lie in the prism zone-. 

 These figures also reveal a vertical plane of symmetry, 

 and an alternating axis c. The action of HN0 3 upon the 

 other forms of the crystal was much like HC1, the 

 scalenohedron remaining unetched. The figures pro- 

 duced by 10-percent HN0 3 upon the various faces will be 

 taken up in order. 



Another crystal, similar to those etched before, was 

 immersed in hot concentrated citric acid for about five 

 seconds, with good results. The rhombohedron was the 

 first form to etch, the 1120 prism immediately after, fol- 

 lowed by the scalenohedron ; the 10T0 prism at this stage 

 had not etched, but did show a slight action of the acid in 

 fine striations or grooves extending parallel to the cleav- 



