Dihescayonal Alternating Type. 211 



acid for 20 minutes ; the results obtained are quite differ- 

 ent ; the figures are quadrilateral forms composed of four 

 distinct faces, triangular in form and meeting at a com- 

 mon point in the center of the figure (see fig. 6). The 

 outer contour is represented by a very fine line more or 

 less irregular and indistinct. The figures have little 

 depth, are blunt at one end, acute at the other and are 

 elongated horizontally. The figures are asymmetric and 

 asymmetrically placed. By heating the acid used in this 

 experiment, and immersing another crystal for two min- 

 utes, very peculiar S-shaped figures were produced, which 

 are composed of two and four faces — those possessing 

 but two faces appear to be the primitive forms ; they 

 possess no plane lying at the bottom of the pit, but the 

 two faces composing the figure meet to form an S-shaped 

 line, whose general direction is inclined to the prism 

 edges. The more mature figures reveal four planes, tri- 

 angular in shape, meeting as a spiral at the center (see 

 fig. 7). The figures are elongated and are inclined to the 

 prism edges. Canals or grooves are also present on this 

 crystal, which are composed of a series of etch figures in 

 parallel arrangement, elongated in a direction 90° to the 

 direction of the groove ; these canals meet the prism 

 edges at an angle of 45°. 



Immersion in hot concentrated citric acid for one min- 

 ute gave still different figures (see fig. 8). These are 

 quite regular, and bounded by four triangular faces, 

 which are arranged in pairs, the two larger faces occupy- 

 ing similar positions above and below, the two smaller 

 ones lying adjacent above and below, so that the figures 

 are slightly bulb or leaf-shaped, and bounded by four 

 curved lines, the two longer ones meeting to form the 

 sharper end of the etching. The longer diagonal varies 

 but a few degrees from a vertical position to the prism 

 edges, while the shorter one runs nearly parallel to the 

 edge of the prism. Although these figures are more sym- 

 metrical in form and position than any described on this 

 (1120) form, they do not conform strictly to any planes of 

 symmetry, and are so placed on adjacent prism faces as 

 to indicate an alternating c axis ; thus they accord with 

 the type. The canals cutting the face are very similar to 

 those previously described. Various exceedingly dilute 

 solutions of this acid were used, but they did not etch the 

 crystals so satisfactorily, due to the easy solubility of the 



