r891.] Mineralogy and Petrography. 145 
graphic directions, and different along unequivalent directions, 
Further, the author finds that elevations due to etching (aetzhiigel) 
occur on faces least capable of resisting solution, while depressions 
(aetzgriibchen) are produced in the least soluble faces. Etching zones, 
he defines as those containing the planes with the greatest capacity for 
resisting solution. Many more results of interest are contained in the 
paper, the character of which is sufficiently indicated by the conclusions 
above referred to. The natural etched figures on the topaz of San 
Louis Potosi, Mexico, correspond in symmetry with the faces on which 
they occur, with the exception of those on the brachypinacoid 2P%, 
which are unsymmetrical. According to Pelikan ™ they resemble the 
figures produced by Baumhauer upon treating the mineral with molten 
potassa. Dufet” obtains 1.54421 as the value of the refractive index 
of quartz, based on the examination of seventeen different specimens 
of the mineral. 
Miscellaneous.—The cosmic dust (kryokonite) collected by Nor- 
denskjéld in Greenland, in 1883, has been submitted to Wiilfing *! for 
investigation, by whom it has been found to consist in greater part of 
feldspar, quartz, mica, and hornblende. There are present in it also 
garnet, zircon, magnetite, augite, and sillimanite, and with them is 
mixed a nitrogenous organic substance. The most interesting con- 
stituents of the dust are little chondri of opaque, isotropic transpar- 
ent, and double refractive material. The larger part of the dust is 
thought to be a sediment from the air, and to have been obtained by 
‘it from a region of crystalline schists. The chondri, on the other 
hand, are thought to be of cosmic origin, since they are similar to the 
chondri obtained in deep-sea soundings. If the amount of the dust 
time is 125 million kilograms, equivalent to a cube of thirty-one yards 
on a side. A new crystal refractometer has been devised by 
Czapski. Its construction and use is carefully described by 9 
inventor in a recent paper in the Neues Jahrbuch ——That — 
relation exists between the habits of crystals of certain minerals and 
their mode of formation has long been recognized, but it has been left 
for Arzruni to undertake a systematic study of this relation. In a 
-= SIb, XE, 1890, p. 331. 
- 2 Bull. Soc. Franc. d. Min., XIII., 1890, p- 271- 
21 Neues Jahrb. f. Min., etc., B. B., VII., p. 152. 
2 Neues Jahrb. f. Min., etc., B. B., VII., p. 175. 
B Zeits. f. Kryst., XVIIL., 1890, p. 44- 
