Mr. L. Fletcher's CrystallograpMc Notes. 477 



present any hernihedral development, they nevertheless find 

 some specimens to be thermoelectrically positive and others 

 negative. 



The bearing of this remark will be more obvious when it is 

 recalled to mind that the investigation of Schrauf and Dana 

 was undertaken with a view to test the accuracy of the induc- 

 tion made by G-ustav Rose from his experiments on iron 

 pyrites and cobaltine* — that the opposition in the thermo- 

 electric behaviour of different specimens of the same mineral 

 is due to the diplohedral hemisymmetry, — and of his inference 

 that the faces observed on the positive and negative crystals be- 

 long only apparently to the same, really to complementary, 

 semiforms. The observation of Schrauf and Dana, that the 

 same thennoelectrical peculiarities are shown by a holohedral 

 mineral of the same crystalline system to which iron pyrites 

 and cobaltine belong, will clearly not strengthen the position 

 taken up by Eose. 



It is therefore of some interest to find that the specimens 

 in this Collection prove that not only is Skutterudite undoubt- 

 edly hernihedral, but that it presents the particular variety of 

 hemisymmetry which is so characteristic of both iron pyrites 

 and cobaltine. 



Brezinaf has pointed out that the opposition of thermo- 

 electric properties shown by various specimens of iron pyrites 

 and cobaltine cannot be satisfactorily explained by diplohedral 

 symmetry, and holds that haplohedral hemisymmetry must be 

 at the same time present — that, in other words, the crystals 

 must be tetartohedral in structure ; and, assuming that the 

 differences in the specimens are only differences of molecular 

 grouping, Brezina regards iron pyrites and the allied species 

 as being really tetartohedral — a view which as yet there is 

 little, if any, crystallographic evidence to support. It seems, 

 however, more probable that Schrauf and Dana are right in 

 attributing this difference, not to crystalline hemisymmetry, 

 but to slight differences of composition or of density ; and the 

 force of their argument will not be very much weakened by 

 the reference of the symmetry of the crystalline forms of 

 Skutterudite to a hernihedral type. 



It may be useful to remark that attention was first directed 

 to the hemihedry by the discovery of a crystal of Skutterudite 

 among the specimens of that typically hernihedral mineral 



* "Ueber den Zusaimnenbang zwischen liemiedriscker Krystallform 

 und tliernio-elektrischen Verhaften beim Eisenkies und Kobaltglanz," 

 Pogg. Ann, vol. cxlii. p. 1 (1871). 



f " Ueber die Symmetrie der Pyritgruppe" Tschermak's Min. Mittheil, 

 p. 23(1872). 



