416 PROCEEDINGS OF THE AMERICAN ACADEMY. 



number of such isomorphic groups whose etch-figures are known, and in 

 every case thei-e is this similarity among the figures. Thus the experi- 

 ments of Baumhauer on the double sulphates,* the lime and strontium 

 hyposulphates,! ^he alums, $ the phosphate (arsenate) group,§ and the 

 apatite family ; || the studies of Baumhauer and Becke on the carbonates 

 of iron and magnesium ;1[ those of Wulff on the barium, strontium, and 

 lead nitrates,** and Becke's on spinel and its relatives ; tt all exhibit a 

 marked stability in the form of the etch-pits as the different members 

 of each group are attacked by the same reagent. The earlier objections of 

 Arzruni are considered to have been met by more recent observations of 

 Baumhauer (Res. der Aetzmethode, p. 40). Retgers does not contend 

 that similarity of etch-figures implies isomorphism, as in the two cases of 

 calcite and soda nitrate on the one hand, and the rutile-zircon-cassiterite 

 group, on the other; but regards the converse as a fixed law, "das 

 erhiirtete Gesetz," that genuinely isomorphic substances always show, on 

 corresponding surfaces, similar etching phenomena. He further states 

 that "a successful study of isomorphism, without continual controls by 

 means of the methods of etching, is no longer conceivable." tt At the 

 same time, he remarks that the etch-figures do vary, and that the limits 

 of variation in most groups have not yet been determined ; they are, 

 however, as far as known, always narrow limits. 



Now, if this contention be valid, the application of the princijile to 

 the amphiboles will have important consequences. It will be remem- 

 bered that tremolite, actinolite, richterite, and astochite, from many 

 different localities and of various chemical composition, gave uniformly 

 the same etch-pits on (110), and, where the material was at hand to 

 determine the point, the same figures on (010), (100), and (101). With 

 respect to the same planes, the much greater group of common and ba- 

 saltic hornblendes, also of very variable composition, agreed among them- 

 selves as well as with barkevikite, glaucophane, crossite, and riebeckite ; 

 but each of these two sets of etch-figure types was so strikingly different 



* Resultate der Aetzmethode, p. 46. 

 t Zeit. fiir Krystalloszraphie, 1877, Bd. I. p. 54. 



t Ber. der k. bayr. Akad. d. Wissenschaften, 1874, and Res. der Aetzmethodcj 

 p. 45. 



§ Resultate der Aetzmethode, p. 43. 



II Ibid., p. 39, and earlier references therein. 



1 Ibid., p. 67. 



** Zeit. fiir Kryst., 1883, Bd. IV. p. 142. 



tt Min. und petr. Mitth., 1885, Bd. VII. p. 200. 



U Zeit. fur phys. Chemie, 1896, Bd. XX. p. 528. 



