468 Ichikawa — Etched Figures of Japanese Quartz. 



Fig. 16 is a model figure showing varieties of the vicinal 

 faces on the positive and negative rhombohedrons of left-handed 

 quartz and the relation between their outlines to the edges on 

 the positive and negative rhombohedral faces. Fig. 17 shows 

 those on the rhombohedron of right-handed quartz. 



Fig. 18 gives the vicinal faces on a rhombohedron of a 

 Dauphine twin of left-handed rock crystal from Takemori ; 

 the character of the rhombohedron is determined by hydro- 

 fluoric acid. Fig. 19 shows the relation between regular vicinal 

 faces and natural depressions on a rhombohedron of left-handed 

 quartz, and also the relation between their outline and the 

 edges on the rhombohedron face. 



In each of the above figures the symmetry of the vicinal 

 face on the rhombohedron of the quartz crystals corresponds to 

 the trapezohedral symmetry," and it also reveals the distinction 

 between positive and negative forms and the difference 

 between the left-handed and right-handed crystals ; therefore, 

 the vicinal faces are a profitable subject for investigation of 

 quartz. 



In figs. 14 and 15, the natural etched figures do not reveal 

 the distinction between the positive and negative rhombo- 

 hedrons, but the vicinal faces show this distinction. The 

 vicinal faces of Japanese quartz are found in smoky quartz 

 rather than in rock crystal, and the faces with them have a 

 stronger luster than those without. 



5. Theoretical Figures showing the Molecular Structure of 

 Bock Crystal (with Plate VII). 



Theoretical figures, showing the molecular structure of rock 

 crystal, have already been described by Lord Kelvin* and Dr. 

 S. Nakamura.f In 1910, I also made a model of the crystal 

 molecule corresponding to the trapezohedral form developed 

 by hydrofluoric acid, and also investigated the molecular struc- 

 ture ; the theory of this is illustrated in the following plate : 



Plate VII. 



Fig. 1 shows the various types of crystal faces occurring 

 on rock crystal and the relation between them ; the striations 

 in the center of the figure give the position of the four trape- 

 zohedrals derived from the dihexagonal pyramid. The faces of 

 these forms are commonly seen to the left or right of the pris- 

 matic edges, corresponding to the positive lateral axes, but the 

 prismatic edges of the negative lateral axes do not have these 

 faces (see Plate II, figs. 1 and 2, A). A gives the left-handed 

 faces, and B the right-handed. 



* Baltimore Lectures, pp. 602-642, 1893. 



t Kigakukuwaishi, vol. iii, No. 13, p. 12, 1910. 



