(502 SCIENTIFIC RECORD FOR 1885. 



(clearly the twinning laiuell«5 as contrasted with the rest of tlie surface, 

 and then strongly heated. Various sections were taken, but in every 

 case when the heating commenced the surface under examination 

 showed a peculiar movement, leading as the temperature rose to a 

 change in the number, position, and dimensions of the twinning lamellae. 

 Finally, when the heat was sufficiently increased all these lamellae and 

 all breaks in the surface disappeared, and the entire surface reflected the 

 light evenly. With a fall of the temjjerature, however, the irregular- 

 ities in structure re-appeared, but not so as to reproduce exactly the 

 original section ; that is, the position and number of the twinning lamellae 

 were changed, although the law of twinning remained constant. The 

 significance of such experiments, especially in connection with the dis- 

 appearance of the optical anomalies with a high temperature, is obvious. 



Brauns has followed up a line of investigation earlier undertaken 

 by him (report for 1883) in regard to the double refraction in crystals, 

 which are isomorphous mixtures of salts normally isometric and isotropic, 

 and has developed some interesting facts of this kind. H(^ recognizes 

 the double refraction in such mixtures (as of different alums) as second- 

 ary, but regards the final cause as yet undetermined, only suggesting 

 that the different volumes of the molecules and different powers of re- 

 fraction may play an important part in the matter. The minerals of the 

 spinel or magnetite group have been made the object of a series of etching 

 experiments by Becke, the aim of which was to add to our knowledge 

 of the connection between etching figures and crystalline structure. The 

 same author had earlier studied the same subject in connection with 

 sphalerite and galena. The minerals now experimented upon were 

 maguetite, franklinite, spinel, and linnseite. The etching figures on 

 boracite and cryolite have been anew studied by Baumhauer. 



The following are some of the more important papers* on general 

 physical mineralogy which cannot be briefly summarized, and hence 

 are alluded to only: Miigge on change of position of crystalline planes, 

 brought about by secondary twinning ; Liebisch on the total reflection 

 in doubly-refracting crystals; Voigt on the values obtained for the 

 constants of elasticity of rock salt and fluor spar, and also on the ex- 

 planation of the pleochroic phenomena of crystals ; Beckenkamp on the 

 methods of experiment in the determination of the constants of elas- 

 ticity ; Wulff on the effect of different kinds of movement upon crys- 

 tallization; Blasius on the expansion of crystals with heat, and the 

 same author on the decomposition figures of crystals; Schrauf on the 

 double refraction of calcite ; Curie on the formation of crystals and the 

 capillary constants of their different faces; Dufet on the variation of re- 

 fractive indices with heat; Danker on the experimental verification of 

 the laws of total reflection deduced from Fresnel's laws of double refrac- 

 tion; Riecke on the pyro-electricity of tourmaline. 



*For fnll titles, see the bibliogra])liy at the close of the chapter. 



