DALY. — ETCH-FIGURES ON AMPHIBOLES. 427 



on the cleavages and on the clinopinacoid of amphibole at least, that the 

 action of hydrofluoric acid is seriously affected by its mixture with j^ure 

 water and also seriously affected, in the opposite sense, by its mixture 

 with sulphuric acid; the action of pure hydrofluoric acid gas is inter- 

 mediate to that of both kinds of mixture. Is it not always necessary to 

 guard the conditions of attack when etching of crystals by chemical cor- 

 rosion is to be the means of comparison among the substances represented 

 thus in the crystalline state ? 



The relatively rapid process of studying etch-flgures in vertically in- 

 cident light can, under the circumstances just outlined, lead to results of 

 importance, not inferior to that attaching to problems where the rather 

 laborious method of determining the exact symbols of figure-faces by 

 means of the goniometer and the Brewster light-figures is necessary. 



(2) A scale of optimum exposures for (110) under standard conditions 

 is recorded and the attempt is made to systematize the amphiboles as 

 regards their attackability on the same face. The comparative attack- 

 ability by hydrofluoric acid of the different faces on certain non-aluminous 

 amphiboles has been determined. It has been found that this property 

 is affected, in large measure, by the physical state of the specimen 

 attacked. 



(3) In many instances it has been exemplified that a systematic com- 

 parison of etch-figures on different species will be of most service if the 

 observer recognizes the principle that there is a decided change in the 

 etch-pit characteristic of any face, in accordance with the stage it has 

 reached in the process of maturing from an initial figure to the often very 

 different figure peculiar to an advanced stage of corrosion. Chiefly for 

 this reason, it has been found difficult, if not impossible, to tell which of 

 the many successive figure-faces composing a given pit on amphibole are 

 the " primary " figure-faces of Becke's definition. It may have been, too, 

 partly for this reason, that I have found it as yet impossible to co-ordinate 

 perfectly the figure-faces and the related directions of easy and difficult 

 solution, so as to construct the " Losungsoberflache " characterizing any 

 amphibole. 



(4) The curious adventitious etch-hills on hornblende, illustrated in 

 Photograph 12, have led us to suspect that they in no wise represent the 

 true cohesional property of the face considered, but have suggested the 

 hypothesis (following Becke) that they may be due to the unequal pro- 

 tection of the mineral surface by the solid products of the chemical reac- 

 tion, and that the parts so shielded from attack will project above the 

 general surface after corrosion has further advanced. It would be 



