Etching a Sphere of Quartz with hydroflnoria acid. 159 



the etching was in all cases the same, and as qiiartz is dissolved by 

 the acid very slowly it is not probable that slight changes in the tem- 

 perature or strength of the acid would have made any appreciable 

 difference. On the ordinary quartz combination of prism, ni, I, lolo, 

 positive rhombohedron r, 1, lOii and negative rhombohedron 2,-1, 

 0111 the following etchings are very easily developed. The positive 

 rhombohedron r yields most readily to the action of the acid becom- 

 ing covered with elongated unsymraetrical depressions having a hori- 

 zontal direction, the heaviest part being to the right in a right 

 handed crystal, fig. 1, plate I, and to the left in a left handed crystal 

 fig. 2, plate I. The top and middle edges of these depressions are 

 nearly straight, the bottom slightly curved, the widest end is ter- 

 minated by a straight edge having the direction of the zonal edge 

 between r and the adjacent z face. These etchings are distributed 

 thickly over the r faces, and although they are not all exactly alike, 

 their general character is well represented in figs. 1 and 2. The 

 effect of this action is also to eat away and replace all of the edges of 

 the crystal toward which the heaviest ends of the etchings are turned ; 

 thus in a right-handed crystal between r and r (lOll and ilOl), r and 

 z (lOll and OlTl) and r and m (lOll and Ollo) all to the right, while 

 the corresponding edges to the left toward which the points of the 

 depressions on r are turned, are left perfectly sharp, except of course 

 the upper parts where r, loli forms a short edge with the adjacent 

 r, on 1 face to the left. In a left-handed crystal, this same phenomena 

 can be observed only with the corresponding edges eaten away to 

 the left instead of to the right. This replacement of the edges is not 

 shown in figs. 1 and 2, but is shown in the original figures of Leydolt, 

 who also determined the symbols of the faces replacing the different 

 edges. According to our experience the replacement of the edges 

 appears more like an accumulation of little facets, all reflecting the 

 light simultaneously, than a replacement made by a single face and 

 for a discussion of the symbols of the faces and the determination of 

 the twinning structure of quartz as shown by the etchings we refer 

 our readers to the original paper of Leydolt. If the crystals are left 

 in the acid for a sufl&ciently long time the edges between the rhombo- 

 hedron faces become so far eaten away that nothing is left of the 

 original rhombohedron faces and the prism is left terminated by the 

 etching faces alone, which flatten out the crystal very much in the di- 

 rection of the vertical axis. 



On the negative rhombohedron z^ the etchings are of an entirely 

 different character, composed of a system of shallow depressions 



