On certain large Crystals of Enstaiite. 379 



ever, to be considered that this action is always towards the 

 interior of the substance, and that the induction of electricity 

 depends on the difference between the disturbances at the two 

 positions at which the radiant cuts the surface of the body, 

 which difference arises from the variation of the intensity of 

 the action according to the law of the inverse square. It is 

 evident that, although the radiants may be effective in produ- 

 cing motion of the atoms in the superficial stratum by reason 

 of the variation of atomic density that subsists there, they may 

 still be of an orderw hich, like light- and heat-radiants, produce 

 no movement of the body as a whole. 



Cambridge, October 17, 1876. 



XLYI. On certain large Crystals of Enstatite found by W. C. 

 Brogger and H. H. Reusch at Kjorrestad near Bamle, South 

 Norway. Memoir by W. C. Brogger of Christiania and 

 G. V03I Kath of Bonn*. 



[Plate IV.] 



• 



THERE are few minerals which offer an equal interest, 

 from the point of view of the progressive development 

 of our knowledge of them, with the two magnesian silicates, 

 enstatite, MgSi0 3 , and olivine, Mg 2 Si0 4 . It is well known 

 that our knowledge of the latter mineral, in proportion as it is 

 increased, has recognized* its abundance and importance. At 

 first found almost exclusively in volcanic rocks and in small 

 crystals, olivine has since been met with in plutonic and meta- 

 morphic rocks, the " serpentine crystals " of Snarum having 

 been recognized as pseudomorphs after olivine. This mineral 

 further acquires a really universal importance through its pre- 

 sence in meteorites. 



Nearly the same may be said of enstatite. Its distribution and 

 its importance have hitherto been less recognizable than in the 

 case of olivine. Professor Kenngott, nearly twenty years ago, 

 gave the name of enstatite to a mineral from Mount Zdjar in 

 Moravia, the composition of which von Hauer determined as 

 that of a normal silicate, MgSi0 3 {Akad. Wien, vol. xvi. p. 162, 

 1855). Professor Des Cloiseaux showed by means of optical 

 researches that the crystalline system of enstatite is rhombic, 

 and established the difference between augite and enstatite 

 (Bull. Soc. Geol. vol. xxi. p. 105). Professor Rainmelsberg 

 first proved by chemical analysis that enstatite is a constituent 

 of meteorites, the stone of Bishopsville (Jfonatsber. Akad. 



* Communicated by the Crystallometric Association. Received Octo- 



