160 Meyer and Penfield — Residts obtained by Etching a Sphere 



with curved contours, giving a sort of feather-like marking with the 

 direction of greatest action turned toward the heaviest etching on 

 the positive rliombohedron, figs. 1 and 2, plate I.* 



The prismatic faces are much less acted upon than the rhombo- 

 hedron faces, the etchings varying somewhat in character but con- 

 sisting essentially of four sided depressions with long and short 

 vertical edges parallel to the edges of the prism, one straight steep 

 edge on the side of the positive rhombohedron r and parallel to the 

 zonal edge between m, and 2, and a shorter slightly curved edge on 

 the side of the negative rhombohedron z. These etchings have 

 definite relations to the symmetry of the crystals and are of reverse 

 character on right, fig. 1, plate I, and left, fig. 2, plate I, handed 

 crystals. On adjacent prismatic faces, the longer or shorter vertical 

 edges are turned toward each other, and by prolonged etching the 

 alternating prismatic edges, toward which the shorter vertical edges 

 of the etchings are directed, are slightly eaten away while the other 

 prismatic edges remain sharp and perfect. 



From a consideration of the above we can now more readily under- 

 stand the action of hydrofluoric acid on a sphere cut from a simple 

 quartz crystal. A sphere of about 2'tt4 c. m. diameter was purchased in 

 New York, and etched by placing it in a lead crucible containing rather 

 a strong commercial hydrofluoric acid, such as can be bought in rubber 

 bottles from dealers in chemicals. The exact strength of the acid 

 was not determined. No special care was taken to place the sphere 

 in any particular position in the acid, its position being accidentally 

 changed nearly every day when the acid was removed, and the solu- 

 tion of the quartz going on so slowly that the acid had a chance to 

 act apparently equally on all similar parts of the sphere. During 

 the progress of the etching, which was carried on slowly in the cold, 

 photographs of the etched sphere were obtained at three stages, which 

 seemed well suited for illustration. 



* According to my experience these etchings on the rhombohedron faces furnish 

 one of the best methods of showing to a beginner in crystallography that the six faces 

 which usually terminate a quartz crystal, are not the faces of an hexagonal pyramid, 

 and all alike, but are those of a positive and negative rhombohedron. To prepare 

 sections for showing this with a microscope, crystals should be etched till the mark- 

 ings are sufficiently distinct, tlien by cementing the crystal, with the etched face down, 

 to a glass plate with Canada balsam and cementing glass plates on either side, the 

 quartz can be ground away witli emery till the glass plates form a large wearing sur- 

 face and the quartz is ground to just tlie thickness of the glass plates ; tiien after re- 

 moving the slice of quartz and cleaning it, it can be cemented to an object glass with 

 the etched surface up and is ready for examination with the microscope. — Penfield. 



