244 WRIGHT—CRYSTALLOGRAPHIC PROPERTIES. _[Junel, 
sible to measure the time of attack or exposition of the acid exactly 
to a second. 
Etching acids used were NO,H and HCl. The action of these 
two acids on the domeykite crystals is totally different. The nitric 
acid works energetically and causes a strong development of gas 
which keeps the submerged crystal in constant motion. The hydro- 
chloric acid in contrast attacks the crystals very slowly (even when 
heated), causes no gas bubbles but becomes gradually colored 
yellow. 
Nitric Acid.—With this acid the best results were obtained with a 
cold (15° C.) dilute solution of four parts concentrated nitric acid 
(70.6 %, sp. gr. = 1.426) and five parts water, with time of ex- 
position 10”—20”. On one crystal unusually sharp etch figures were 
observed after an exposition of 13”. A long series of different con- 
centrations were tried (from concentrated NO,H down to 1: 2 and 
lower). The times exposed varied from 10” up to 120”. Nitric acid 
appears to etch most rapidly parallel to the outer edges of the crys- 
tal. The etch figures are very small, remarkably flat and shallow ; 
exhibit generally sharp hexagonal outline, and grade especially on 
too long exposition or too concentrated acid into a hexagonal net- 
work of three systems of straight lines running parallel to the outer 
edges of the crystal. Compare the following figures; 
Fig. 9 (dilution 4:5. Time of exposition 30/’. Magnification 40 x ). 
Fig. 10 ee ihe Ge “c “ec 20/!. “ce to x ): 
Fig. Pr “ss AK 5: “ “ce 4o!/, “ 40 X yi 
Fig. 12 “ ats “ “ 60/’. “ 60 X ). 
On one crystal, however, three-sided figures were observed, their 
slightly convex lines running parallel to the outer edges of the crys- 
tal. On several crystals one of the three systems of lines appeared 
in certain parts of the field to be less strongly developed than the 
remaining two sets, while in other parts of the field a second system 
was absent, etc. (Fig. 11). The rule seems to hold good in such 
cases that in the near vicinity of an outer edge that system of lines 
is poorly developed which runs parallel to the edge. The outer edge 
seems to have had a certain influence on the development of the 
lines of the etch figures. Usually, however, all three sets of lines 
are equally well formed (Fig. 12). The etch figures are so small 
that they give no noticeable reflexion signals on the goniometer. 
Hydrochloric Acid.—Hydrochloric acid attacks domeykite under _ 
