1886.] Mineralogy and Petrography. 159 
cleavage planes, but were intimately related to the symmetry of 
the crystal, and thus opened up a new method of investigating 
opaque minerals. In order to study more closely the connection 
between the figures produced by weathering and those produced 
by etching, Blasius,! of Strassburg, subjected a number of sub- 
stances in crystal form to the action of alcohol, and also placed 
them in desiccators with strong sulphuric acid. As the result of 
a large number of experiments, he concludes that many of the 
figures produced by weathering (including etched figures) cannot 
be brought into close relation with the directions of cleavage or 
the curves of hardness in the substances acted upon. Moreover, 
their shapes differ according to conditions, and, finally, from 
a knowledge of the etched figures on a number of faces the shape 
of those on others can be deduced. F. Becke? adds further to 
our knowledge in an article on the etched figures of minerals of 
the magnetite group. Magnetite, spinel, franklinite and linneite 
were treated with sulphuric, nitric and hydrochloric acids of dif- 
ferent concentrations and during periods of different lengths, and 
besides with alkaline solutions. 1. On all the crystal faces, the 
figures were composed essentially of the same planes of etching, 
the principal planes of etching. 2. The principal planes of etch- 
ing lie in a determinate zone. 3. These planes of etching offer 
the greatest opposition to the action of solvents. 4. Depressions 
are formed on such faces as belong to the zene of etching, eleva- 
tions on those which lie far without it. 5. Cleavage planes can- 
not, at the same time, be planes of etching. 6. Linnzite, when 
etched with acids, deports itself like magnetite; when treated 
with alkaline solutions, an entirely different plane becomes the 
principal plane of etching. Consequently, it may be assumed 
that “the elemental atoms in the crystal molecule maintain a 
definite position with relation to one another.” In linnæite 
(Co,S,), for instance, “the cobalt atoms are turned toward 
the cubic faces and the sulphur atoms toward the dode- 
cahedral faces,” because when treated with acid the cubic 
faces are dissolved fastest, but when treated with fused potash, 
the dodecahedral faces offer the least opposition to the solvent 
action of this reagent. Baumhauer* makes practical use of the 
method of etched figures in an investigation ofthe character of 
the massive bornite from Chloride, N. M. When a polished sur- 
face of this mineral is treated with nitric acid it breaks up into 
several fields, each of which reflects the light differently, showing 
that the massive material is made up of an irregular intergrowth 
of individual crystals. At the same time the fact is brought out 
twinned inclusions of chalcocite and chalcopyrite are not 
ommon. 
i Zeits. für Kryst. und Miner., X, p. 221. 
3 Min. und Petrogr. Mitt., VII, p. 195. 
Zeitschrift für Krystallographie, X, p. 447- 
