DALY. — ETCH-FIGURES ON AMPHIBOLES. 409 



with the rotation, there is a simultaneous distortion of the ontline so as 

 to make the angle B A D more and more acute. The latter could not be 

 measured closely in the cases of dilution of 50 per cent or more on ac- 

 count of the poor development of the pits. The longer edge preserved 

 its distinctness much better than did the shorter edge ; hence the angle 

 A D If was measurable throughout. 



The light figures remained quiescent, neither changing in shape nor 

 orientation, so far as those rather unsatisfactory figures would permit of 

 measurement. In the same manner, I could discover no variations in 

 the pits on the prism-face (HO). 



Bomer has described other examples of the dependence of the results 

 of etching with hydrofluoric acid on its state of dilution in water. He 

 found the form and attitude of the pits on the base of quartz to alter so 

 much with dilution that, whereas concentrated acid gives figure-faces 

 belonging to the right trigonal pyramids, with very dilute acid they are 

 negative rhombohedrons. Intermediate forms characterize decrees of* 

 concentration between these two extremes.* Baumhauer discussed a 

 similar anomalous behavior in the pits on apatite, using hydrochloric acid 

 as the solvent.f There is throughout a close analogy between the pits 

 on (0001) of apatite and those on (010) of actinolite. In both, we have 

 dark fijjures and two cateiiories of liiiht figures, and, finally, the same 

 tendency to rotate with decreasing concentration of the respective acids 

 in aqueous solution. | 



It is well known that sulphuric acid, when added in small quantities, 

 will in certain cases intensify the solvent power of hydrofluoric acid. I 

 made one or two trials of a basaltic hornblende to see whether in this way 

 the figures of corrosion might be improved. They resulted in partial 

 failure, for, although the pits were a little larger than usual, they lost 

 considerably in definiteness of outline. It then occurred to me to try 

 mixtures of the two acids in various strengths on a more resistant sub- 

 stance, actinolite, primarily to determine what, if any, would be the 

 influence of the sulphuric acid on the etching. The problem was anal- 

 ogous to that just discussed for dilution with water, the procedure was 

 similar, the results just as striking. The table is so like the last as to 

 need no special explanation. (Figure 7.) 



* Neues Jahrbucli fiir Min., etc., 1891, Beil. Bd. VII. p. 535. 

 t Eesultate der Aetzmetliode, p. 48. 



t Becke states that concentration affects the position of the figure-faces in the 

 etch-zone. Min. u. petrog. Mittheil., 1883, Bd. V. p. 487. 



