646 dCIENTIFIC RECORD I'OR 1884. 



subject would seem to be a little overdone when two hundred and 

 seventy-five pages are given to its discussion. Schuster, after a very 

 careful comparison of the angles measured by him on the Swiss crystals, 

 confirms the conclusion of Hintze that they have almost identically the 

 same axial relations as the crystals from Kussell, N. Y. Of other 

 papers devoted to a similar class of subjects may be mentioned one by 

 P. W. von Jeremejew on Eussian linarite (abstract in Zeitsch. Kryst., 

 IX, 430) ; another by C. Morton on the stephanite of Kongsberg, Nor- 

 way {(Efversigt K. Yet. Akad. Fork., February 13, 1884), on the am- 

 phibole of the Aranyer Berg by Franzenau (Zeitsch. Kryst., viii, 568), 

 on epistilbite by Hintze (ib., p. 605), on andalusiteand topaz by Griinhut 

 {lb., IX., 113). Solly has described {Min. Mag., vi., 80) a crystal of 

 tourmaline from Pierrepont, N. Y., which showed a tetartohedral devel- 

 opment of the scalenohedral planes ; the matter needs confirmation. 



Hj. Sjogren has settled {(Efv. E. Vet. Alad. Fork., April 9, 1884) 

 finally, it would seem, the disputed crystalline form of graphite, a species 

 which has generally been referred to the hexagonal system in accord- 

 ance with the early observations of Kenngott, but which was afterwards 

 made monoclinic by Nordenskjold. Finding the direct determination of 

 the form by accurate measurement impracticable, because of the imper- 

 fection of the crystals, he attacked the problem indirectly. He shows 

 that the etching figures and the figures produced by partial combustion, 

 also the isothermal curves, all agree with the hexagonal system. 



The subject of the double refraction of minerals, isometric in geomet- 

 rical form, has received some important contributions during the past 

 year, and we now seem to be approaching to a full understanding of 

 the true relations, A few years ago it appeared as if the only species 

 of those formerly included in the isometric system, which were likely to 

 be allowed to remain there, were those which from their opacity (like 

 galena) did not allow of an optical examination, so general a phenome- 

 non had this anomalous double refraction been shown to be. The new 

 light recently thrown upon the subject has come from the observation 

 of Mallard, that boracite loses all its double refraction and becomes 

 isotropic at a temperature of 265° C. To appreciate the significance 

 of this it is necessary to recall the fact that while the geometrical 

 form of boracite crystals is in strict accordance with the require- 

 ments of the isometric system, sections examined optically uniformly 

 show double refraction, the section being seen to be divided, when ex 

 amined in polarized light, into parts more or less regularly arranged, 

 with different optical orientation. Instead of assuming, then, that the 

 crystalline form is only a case of pseudo-symmetry, an imitative, or " mi- 

 metic " (to use Tschermak's word) form due to the comj)lex twining ol 

 a number of orthorhombic crystals, we must conclude that the isomet 

 ric form was the original one, and that the conditions under which 

 the crystals were formed were similar to those we now obtain by an eleva 

 tion of temperature to 265^ 0. Under these conditions of formation then 



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