164 Meyer and Penfield — Mesults ohtahied by Etching a Sphere 



being impossible to reproduce the delicacy and beauty of the mark- 

 ings as they appeared on the perfectly transparent material of the 

 quartz. 



Pyro-electrrical experiments. — To further test the relation of the 

 sphere to crystallographic axes, it was heated for some hours in an 

 air bath to 100° C, and on cooling dusted with a mixture of red oxide 

 of lead and sulphur, according to the excellent method described by 

 Professor A. Kundt of Strassburg.* The red oxide of lead and 

 yellow sulphur arranged themselves in six alternating vertical bands 

 about the equator of the sphere, the red bands being located midway 

 between the heaviest parts of the etchings on the positive rhombohe- 

 drons above and below. On the etched sphere represented in figure 

 2, plate II, a red band ran vertically a little to the right of the cen- 

 ter, a yellow one a little to the left of the center and so on, three red 

 alternating with three yellow about the equator of the sphere. On 

 the etched sphere represented in figures 5 and 6, plate II, the slightly 

 attacked parallelogram parts, representing the ends of the lateral 

 axes, were yellow while the center of the sharp edges midway be- 

 tween them were red. These pyro-electrical phenomena, according 

 to B. von Kolenko,! indicate that the crystals from which our sphere 

 was cut was a right-handed one ; that part of the sphere where neg- 

 ative electricity develops on cooling and where the positively elec- 

 trified red oxide of lead deposits, indicating the position of the pris- 

 matic edge, where the right-handed trapezohedral faces above and 

 below would occur. The even distribution of the pyro-electricity 

 into six alternating positive and negative sections prove the simple 

 character of the sphere and the absence of twinning. This latter 

 was very important for the success of our experiment, lor if the 

 sphere had been cut from a complicated twin crystal the etchings 

 would have arranged themselves in a very confused manner, and the 

 shapes produced by the etching would have been very much modified. 



In closing we wish to express our thanks to Mr. J. M. Blake of 

 New Haven, for the care which he took in photographing such a 

 difficult, transparent object, and to Mr. E. Bierstadt of New York, 

 for the pains which he took in the preparation of the plates, 



Mineralogical Laboratory, Sheffield Scientific School, Feb. 4th, 1889. 



* Ann. d. Phys. u. Chera., 1 883, xx, p. 592. 

 t Zeitsch. f. Kryst, ix, 1884, p. 1. 



