THE DISINTEGRATED SURFACES OF CRYSTALS. 273 



the acute solid angles, and replaced it hj a plane inclined 71° to a face of the 

 rhomb, the common section being parallel to the long diagonal of this last face. 

 After being immersed in dilute nitric acid, it gave the strange branching figure 

 shewn in Fig. 27, where abc forms the brightest portion. I obtained the same fio-nre 

 with another crystal, but the parts xy were wanting, and b and c were continued 

 through a to m and n. The side a was directed to the obtuse angle of the rhomb. 

 With another crystal, in which the artificial face was inclined 104° instead of 71°, 

 the figure shewn in Fig. 28 was produced. 



My next expermients were made with sulphate of potash, a crystal which be- 

 longs to the rhombohedral system. By the slightest action of water upon the flat 

 summit of a hexagonal prism, it produced six luminous unages, symmetrically 

 arranged round the central image, each image being opposite a side of the hexa- 

 gon. All these images were connected with the central image by a halo of fainter 

 light. The faces of the hexagonal prism produce the figure shewn in Fig. 29, the 

 line AB being coincident with the axis of the prism. By continuing the action 

 the branches C, D vanished, and the figure appeared as in Fig. 30, the imao-es be- 

 ing connected with a haze of light. 



A more remai-kable effect was produced with the faces of the truncated pyra- 

 mid. Three of the six faces produced the effect shewn in Fig. 31, whUe the other 

 three alternate faces produced the same figm-es, but without the wings E, E. Some- 

 times two images are seen below B. 



My attention was now directed to the system of crystallization in which the 

 base of the primitive crystal is a square. Having immersed a fine crystal of Faroe 

 apophyUite in dUute nitric acid, the summits of the prism were alone acted upon. 

 They produced a figm-e with/oM/- rectangular radiations directed to the angles of 

 the summit, and foiu- much shorter ones pointing to the sides of the square sum- 

 mit. The four large rays appeai-ed first connected with a luminous web, and the 

 four small ones were subsequently developed. 



The very same figure, but with some modifications, was produced by the ac- 

 tion of water upon the summit of the square prism of sulphate of potash and cop- 

 per. The small radiations were produced last, as in apophyllite, but what is re- 

 markable, they are dn-ected to the angles, and not to the sides of the square face. 

 The extreme solubility of this salt renders it difficult to develope the figure dis- 

 tinctly. 



From another crystal of the same class, superacetate of copper and lime, I ob- 

 tained the beautiful figure shewn in Fig. 32, where the eight radiations are of 

 equal length, and the images at their extremities connected by beautiful curves 

 of light concave outwards. 



I have made a great number of experiments with crystals belonging to the 

 prismatic system, such as, sulphate of magnesia, borax, tartrate of potash and soda. 



