408 PROCEEDINGS OF THE AMERICAN ACADEMY. 



second typical pit manifestly of younger generation, but with its A B 

 steeper than A D. Brauns found on the etched surface of sylvite neigh- 

 boring pits bounded by figure-faces of different steepness, but he does 

 not seem to have connected the phenomenon with the difference of 

 maturity of the two pits. (See Nuues Jahrbuch fur Miu., etc., 1889, 

 Bd. I. p. 113.) 



Either of these types of dark figures may show a keel at the intersec- 

 tion of a pair of figure-faces (Plate I. Fig. 23 b). 



It was in connection with the study of the pits on (010) of actinolite 

 that I became convinced of the necessity of fixing a standard degree of 

 dilution of the hydrofluoric acid before beginning a series of comparative 

 experiments in etching the araphiboles. The problem could be here 

 more successfully attacked than in the examination of the figures on 

 (110) because of tlie greater likeliiiood of being able to observe differ- 

 ences in the shape or arrangement of the systematically straight-edged 

 pits on (010). Six cleavage pieces of Zillerthal actinolite were immersed 

 in II F, either in the form of pure gas or in different states of dilution 

 with water. The procedure and the results are synopsized in the accom- 

 panying table, which shows the effects on the angles A D H 3ind BAD 

 of the dark figures under the different conditions (Figure 6).* 



Specimen. Solvent. Exposure, 



No. Minutes. 



(1) Pure HF (gas) 70 



(2) Com. cone, water solution of HF 2\ 



(3) 75% HF, 25% water 4 



(4) 50% HF, 50% water 15 



(5) 25% HF, 75% water 25 

 (G) 10% HF, 90% water 20 -10 



No. 1 was etched by hanging it above the surface of some concentrated 

 water solution of the acid, that was very gently heated far below its 

 boiling point, and thus only a small percentage of water vapor could be 

 present during the reaction. The other examples were etched in the 

 ordinary way on a water bath. The percentages of dilution are by 

 volume. 



The effect of dilution with water is, then, to produce a rotation of 

 each dark figure about a line perpendicular to the crystal plane. The 

 direction of the rotation is opposite to that of the hands of a watch, its 

 amount (within the limits of these experiments) about 30°. Along 



* For Figures 6 and 7 see page 406. 



