and Crystals of Quartz with hydrofluoric acid. 163 



destroy the original polish of the sphere till toward the very end of 

 the experiment and not enough to appreciably diminish the diameter 

 of the sphere. Although the original diameter was not accurately 

 measured care was taken soon after commencing the etching to cut a 

 hole in a card board very carefully just large enough to allow the 

 sphere to pass and at the conclusion of the etching the quartz just 

 touched at these three points when passed through the same hole. 



The quartz was still exposed to the action of this acid for about a 

 week, but the general eftect was not different from that shown in 

 figures 5 and 6. Of course the sphere was still further flattened in 

 the direction of the vertical axis and the three parts at the extremi- 

 ties of the lateral axes where the acid had acted least, became con- 

 siderably changed, being flattened out more in a vertical direction 

 and therefore appearing as parallelograms, relatively much more 

 elongated horizontally. Figure 3, plate I, is from a photograph 

 taken at this stage in which the relation of these parts to the longer 

 sharp edge between them is less than in figure 5, plate II. At this 

 stage also the etching of the sphere was stopped and the specimen 

 deposited in the collection of Professor George J. Brush, New 

 Haven, Conn. 



In review it will be noticed, as is the case in all etching, that the 

 acid acts very unequally on different faces of a crystal and therefore 

 on different parts of the sphere, equally, however, on those similar 

 parts of the sphere which are similarly situated with reference to 

 hexagonal axes. The action is greatest at the two extremities of the 

 vertical axis. The action seems to be, especially toward the end, to 

 lift off" or dissolve away layers of molecules from above and below 

 while there are three parts, which are each one of the ends of three 

 lateral axes, where the acid exerts practically no solvent action. 

 These parts diminish in size as the action of the acid continues but 

 not by any action of the acid upon them directly, except as the 

 molecules were taken away from above, below and at the sides. 

 This is one of the most striking features of the experiment that in 

 these three directions quartz is almost absolutely insoluble in hydro- 

 fluoric acid. As a study in symmetry the experiment all along was a 

 very interesting one. The etched sphere could never be divided by 

 a plane into symmetrical halves and showed throughout all of the 

 experiment the tetratohedral character of a right-handed quartz 

 crystal. The sphere was cut from a crystal which would have shown 

 etchings like those in figure 1, plate I. The accompanying illustra- 

 tions give only a faint idea of the beauty of the etched sphere, it 



