X. — Results obtained by Etching a Sphere and Crystals op 

 Quartz with Hydrofluoric Acid. By Dr. Otto Meyer and 

 Samuel L. Penfield. 



A few years ago one of us * published the results of an experiment 

 of etching a sphere of calcite with acetic acid in which the symmetry 

 of a calcite crystal was brought out by the character of the etchings 

 on the sphere and the final result of eating away the greater part of 

 the calcite was a crystalline figure with rounded faces, but with a 

 decided steep scalenohedral habit with truncations at the extremities 

 of the vertical axis. This suggested to us the idea of trying similar 

 experiments on spheres cut from other crystals. The difficulty of 

 course lies in obtaining spheres of perfectly pure homogeneous ma- 

 terial ; the results furnish, however, an interesting and instructive 

 means of studying the symmetry of any crystalline substance and as 

 parts of the sphere are parallel to all possible faces of a crystal, as 

 soon as the relation of the sphere to the axes of the crystal is made 

 out the character of the etchings in any particular part of the sphere 

 will determine the character of the etching produced by the solvent 

 on any crystal face parallel to that particular part of the sphere. 

 The ease with which spheres of Japanese quartz can be obtained and 

 the readiness with which quartz yields in certain directions, to the 

 action of hydrofluoric acid, made the following experiments quite 

 easy, while the results as will be seen are far more striking than one 

 would at first suj^pose. 



The results of our experiments will be better understood by re- 

 viewing some experiments made in 1855 by F. Leydolt f on quartz 

 crystals in which he showed that hydrofluoric acid acts very un- 

 equally on the diflerent kinds of faces, so that not only the right and 

 left-handed character of the crystals, but also all the complexity of 

 twinning can be made to appear by etching. The experiments were 

 repeated by us by placing simple quartz cx'ystals from Herkimer, 

 N. Y., in strong hydrofluoric acid and leaving them till sufficiently 

 distinct etchings were produced. In these experiments, some of 

 which were carried on in cold and some in hot acid, the character of 



* Meyer, Jahrb. Min., 1883, i, 74. 



f Sitz-ber. der Wiener Akad., 1855, xv, p. 59. 



