66G SCIENTIFIC RECORD FOR 1883. 



graphic, optical, and chemical relations of these minerals are too intri- 

 cate to allow of their being developed here, but it may be said in 

 general that the result of the large amount of work done upon this dif- 

 ficult group has been to establish definitely the form and composition 

 of most of the members, so that, though the conclusions reached are to 

 some extent contradictory, we are much nearer to a full understanding 

 of the subject than was the case a few years since. 



It has been shown by H. 0. Lewis that the magnetite present in den- 

 dritic forms in many specimens of mica accords in the direction of its 

 markings with the directions of the lines of the " strike-figures" pro- 

 duced by a blow on the mica surface with a blunt point. It is, then, a 

 case of parallelism between the crystalline directions of the two species, 

 analogous to many similar examples, like rutile on hematite, albite on 

 orthoclase, tetrahedrite on chalcopyrite, and so ou. The same author 

 (Proc. Acad. Nat. Sci. Philacl., February, 1883) has described what he 

 regards as a crystallized serpentine from Way's quarry, Delaware. A 

 very much detailed article on the structural peculiarities of the danburite 

 crystals from the Skopi, Switzerland, has been published by Schuster. 

 (Tscliermalc, Min. Petr. Mitth., V, 397.) 



Some progress has been made in the investigation of the relations of 

 crystals with respect to cohesion. Gleitflache is the German name for a 

 slipping surface, or the direction in a crystal in which a slipping of the 

 molecules may be made to take place by pressure or a blow. This sub- 

 ject was first developed by Eeusch, but others have since followed in 

 the same line. The well-known artificial twins of calcite (as shown by 

 Baumhauer) are a good example of this molecular slipping. Recently 

 the subject has been further investigated by Miigge (Jahrb. Mm., 1883, 

 I, 32 ; ii, 13) with respect to gypsum, stibnite, and some other species. 

 More important than his special observations are the conclusions which 

 he reaches that these "Gleitfiaehen" appear to be limited to minerals 

 showing great differences of cohesion in different directions — that is, 

 those Avith perfect cleavage — and they exist most distinctly at right 

 angles to this cleavage direction, or where the cohesion is a maximum. 

 This molecular slipping then takes place because the cohesion being- 

 greatest a separation of the molecules can be effected with most diffi- 

 culty, while the slipping takes place with no more difficulty than in 

 other directions. Planes of cleavage and of molecular slipping are theu 

 diametrically opposed to each other. 



The methods of determining the specific gravity of minerals and of 

 separating mechanically mineral mixtures have been reviewed by 

 Gisevius in an inaugural dissertation published in Bonn. The author 

 concludes that the methods usually employed, involving the rise of the 

 hydrostatic balance, the pyenometer, and the spiral balance of Jolly, 

 are all liable to considerable error. He then goes on to the discussion 

 of a method suggested by Brugelmann, namely, to determine the abso- 

 lute weight of the fragment by a balance, and then its volume by the 



