462 Ichihawa — Etched Figures of Japanese Quartz. 



The symmetry of the dull figures (except in Plate III, figs. 

 5 and 6) on the surface of these etched spheres exhibits the 

 symmetry of the trapezohedral group to which they belong. 



Both poles, in the vertical axis of the etched sphere, are 

 more quickly dissolved away than the equator, and the positive 

 ends of the lateral axes more quickly than the negative ends ; 

 the result is, therefore, a trapezohedral form as in those of the 

 simple crystal of the quartz with bi pyramid (Plate IV, fig. 1, 

 B, compare Plate II, fig. 1, D.). 



A sphere of a regularly developed Dauphine twin reveals 

 the twelve dodecants on its surface by etching, with the verti- 

 cal axis and intermediate axes, and each dodecant is occupied 

 by the positive and negative lateral axes ; therefore when each 

 dodecant belonging to the area of the positive or negative 

 lateral axes of a simple quartz crystal is revolved 60° about 

 the vertical axis, the quartz is supposed to make up the given 

 regular Dauphine twin. 



The etching of basal sections cut perpendicular to the verti- 

 cal axis of twin crystals reveals the revolved or interpenetrated 

 areas and also the direction of the rotation : the etching of 

 such sections is, therefore, an important method of the deter- 

 mination of a twin crystal. 



When the acid is near concentration the section yields trape- 

 zohedral elevations (see Plate III, tig. 11), but if very weak the 

 surface shows the negative crystal of the trapezohedral ridges 

 (see Plate III, tig. 4) ; also if the section is kept in the very 

 w T eak acid, depressions of 7-like shape are formed on the sur- 

 faces of the elevations (see Plate IV, tigs. 11 and 12), and 

 these are gradually twisted to the left or right of the verti- 

 cal axis (see Plate IY, tigs. 6 and 7.) Hence, the phenomena 

 prove that the direction of the molecular dissociation in the 

 etching corresponds to that of the circular polarization. 



In etching hexagonal prisms or columnal seals, etc., of rock 

 crystals it is found that they are attacked rapidly in the direc- 

 tion of the vertical axis, but barely at all in the direction of 

 the lateral axes ; the specimens are, therefore, gradually modi- 

 fied into thinner hexagonal or rounded forms, and at last are 

 wholly dissolved away. 



When etched basal sections, as Plate IV, figs. 11 and 12, 

 etc., are again placed in the pure acid the sections are modified 

 into a translucent, smooth plane by violent action of the acid, 

 but after a day or two the ridges are again formed on the sur- 

 face, and after three or four days 7-like depressions are 

 formed, and after a week these are twisted in a direction to the 

 left or right of the vertical axis. 



The development of etching figures on the poles of the ver- 

 tical axis of a sphere or a basal section, etc., of quartz crystal 



