410 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



Specimen. Solvent. 



No. 



(1) Pure HF (gas) 



(2) Commercial cone, solution 



(3) 95% HF, 5% H2SO4 



(4) 90% HF, 10% H2SO4 



(5) 75% HF, 25% H2SO4 



(6) 50% HF, 50% H..SO4 



(7) 20% HF, 80% H2SO4 



26± Indeterminable. 



The mixtures of acids were proportioned by volume. 



The table clearly expresses a rotation of the figures with increasing 

 percentages of PI2SO4 in a right-handed direction, that opposed to the 

 direction of movement in the water-dilution series. At the same time, 

 the angle BAD grows larger, and thus the parallelogram tends more 

 and more towards a rectangular outline. In both respects, the influence 

 of sulphuric acid is in the shaping and the arranging the pits on the 

 clinopinacoid, when etched by means of a mixture of that acid and 

 hydrofluoric acid, the reverse of that of pure water mixed in the same 

 manner with hydrofluoric acid. I am not prepared to offer any expla- 

 nation of this interesting phenomenon.* 



It may be noted that the surface of cleavage (110) exhibits great 

 changes in the etch-figures as the proportion of the sulphuric acid in- 

 creases. With only 5% of the latter (Plate I. Fig. 24), there is a sen- 

 sible variation iu look from the normal type, and in the 50% solution a 

 strong suggestion of the Wolfsberg figure of common hornblende 

 (Plate I. Figs. 25 a and 25 b). 



Tremolite and richterite afforded, on the clinopinacoid, etching phe- 

 nomena identical with those described for actinolite when exposed to 

 concentrated HF; the last mentioned experiments of mixture were not 

 essayed in connection with them. 



* Baumhauer not only showed a rotation of the figures on the basal plane of 

 apatite with increasing dilution of tlie solvent, hydrochloric acid, but he also 

 established a rotation in the same direction when nitric acid was substituted for 

 the hydrochloric, and a rotation in the opposite direction if sulphuric acid be simi- 

 larly employed. Ref. in Bomer's article, Neues Jahrbuch fiir Min., 1801, Beil. 

 Bd. VII. p. 538. The accumulation of such facts as these makes it difficult to 

 follow Becke in the hypothesis that his " Hauptaetzflachen " have simple indices, 

 because of greater molecular density in planes having such indices. The molecu- 

 lar density evidently does not change with the solvent. (Min. u. petr. Mitth., 

 1887, Bd. VII. p. 200). 



