DALY. ETCH-FIGURES ON AMPHIBOLES. 383 



results. As a matter of fact, I have found (010) and (100) to be among 

 the interesting faces for etching on amphiboles. Moreover, not only are 

 they very common planes ; they are also those most likely to reveal the 

 fundamental features of crystal structure. 



2. Since the production of etch-figures on a bisilicate is in large part 

 the result of a chemical reaction between mineral and solvent, it is clear 

 that the figure will, in every case, depend on the chemical nature of both. 

 Two varieties apparently isomorphous and differing little in composition 

 may afford figures markedly different from each other, although produced 

 on planes with the same symbols and with the same solvent. Examples 

 will be noted in the sequel. Such being the fact, it is natural to conclude 

 that the figures shall be similarly sensitive to small changes in the solvent 

 also.* A striking illustration is to be found in the series of etch-pits 

 formed on (010) of actinolite by hydrofluoric acid in various states of 

 dilution in water or mixture with sulphuric acid. There is a steady 

 change in the orientation of the figure as the state of purity of the hydro- 

 fluoric acid is affected. Details concerning this phenomenon are given 

 in the section devoted to a description of figures on (010). 



I chose hydrofluoric acid as the universal attacking reagent on account 

 of its convenience and efficiency. Inasmuch as its working depends on 

 the degree of concentration of the acid, it becomes necessary to fix on some 

 particular grade of acid. A number of trials soon convinced me that the 

 concentrated commercial water solution is for general purposes the best. 

 Not needing special preliminary preparation, it is easily obtainable in any 

 desired quantity ; experiment showed that it gave the most satisfactory 

 figures just as the alums afford the best results with the solvent in an 

 active state, f There is one danger to be guarded against, namely, the 

 loss of concentration with prolonged heating of the acid ; hence the ad- 

 vantage of easy renewal of the reagent. 



Temperature and its function, the duration of immersion, are now well 

 established to have a strong influence on the process and effects of etch- 

 ing. A new variable must thus be considered. As a result of a large 

 number of trials made both incidentally and with this distinct purpose in 

 mind, I found that good figures could be produced at many different tem- 

 peratures ; thus, Zillerthal actinolite will yield well developed pits of 

 corrosion when boiled one minute in HF, or at three minutes in HF 



* Cf. Von Kobell, Sitzungsber. Miinchner Akad., 1862, p. 199 ; Ben Saude, 

 Ueber den Perowskit, Gottingen, 1881; Meyer, Neues Jahrbuch, 1883, Bd. I. 

 p. 77. 



t Klocke, Zeit. f iir Kryst, 1877-78, Bd. II. p. 130. 



