390 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Having traced tlie influence of chemical composition and of physical 

 conditions on the property of attackability, we may now proceed to in- 

 quire whether the single planes of an amphibole have different powers 

 of resisting solution in hydrofluoric acid. I shall once more use the infor- 

 mation gained in the process of etching ; the appearance of figures and 

 the loss of illumination using vertically incident light will be taken as 

 the criterion of attack. Actinolite, tremolite, and common hornblende 

 were thus examined. 



A crystal of V. 42 was exposed first, 40 seconds, then one minute. 

 At 40 seconds (111) was visibly roughened but showed no figures, while 

 (110) remained practically intact; at the end of the second exposure both 

 planes were equally provided with pits. From the mere development of 

 figures, it would appear that (Til) and (110) were of about equal attack- 

 ability, but the roughening of surface gave much more reliable indications 

 of the fact. Combining with this, the observations on the other faces, 

 the series is correctly written iu the order of increasing resistance to the 



( (100) 1 

 acid (Til), (011)-< (010) V. As a matter of fact, the maturing of pits 



((.no)) 



on the various faces often occurred at intervals so far apart as to permit 

 of a pretty tolerable determination of the facial attackability by means 

 of their study alone. It is essential, of course, to distinguish etch-pits 

 from etch-hills, which may (as described by Becke) simultaneously ap- 

 pear on two different faces of the same crystal. A generalization was 

 made from the study of twelve crystals of aluminous hornblendes, as 

 follows: V. 26, V. 32 (2), V. 33, V. 35, V. 40 (2), V. 42 (3), V. 54, and 

 P. 55. Arranging the planes in order of increasing resistance to the acid, 

 we have, (Til), (Oil), (130), (100), (010), (110). But what has been 

 said about the phenomenon as affecting the corrosion on (110) applies 

 with equal force to the other planes. With V. 54, I obtained the series 

 (TOl), (010), (110), (Oil), in the same order as above, — an anomalous 

 order which I could see was correlated with the lustre and general state 

 of repair of each of the faces belonging to the crystal in question. By 

 reason of the peculiar geological conditions, and of the position of the 

 crystal in its druse, a prism face may suffer alteration while an end face 

 may escape that process and the latter can thus resist an attacking acid 

 longer, though, as we have seen, the terminal planes are regularly the 

 ones to yield first. 



Klocke, Zeit fiir Kryst., 1877, Bd. II. p. 128. Cf. Retgers, Zeit. fur Phys. Chemie, 

 1895, Bd. XVI. p. 638. 



