On Crystallography. »fi • 



a regular octahedron. Nothing is more common in crvstals 

 of sulphuretted iron than this modification, to which we 

 give the name of cubo- octahedral si/lphnretied iron. 



Here the remark again occurs which we made with re- 

 spect to decrements on the edges. Jf we confine our con- 

 sideration to the immediate effects of decrements on the 

 angles of two opposite faces, for example, on those of the 

 bases A E O I, A' E' O' V, (fig. 20) and if we suhseqncntly 

 imagine the eight faces to which these decrements give ex- 

 istence, are prolonged between the bases to the point of 

 intersecting each other, the result will always be a reii^ular 

 octahedron, supposing that the decrements attain their 

 limit. 



If the law of these decrements followed a more rapid 

 course, /. e. if more than one course was subtracted, then 

 the three trapezoids s t o x, m t or, n r o x, (fig. 25) which 

 would be formed around the same solid angle, would no 

 longer be on a single plane; they would incline towards 

 each other, and the secondary solid would have twenty-four 

 faces wiiich would also be trapezoids, but with angles of a 

 difl'erent measure. 



The trapezoidal analcime* has a structure of this kind 

 which depends on a decrement l)y two courses on all the 

 angles of the primitive cube. The form which results is 

 completely similar to that of the trape^.idal garnet ; but 

 this resemblance is only exterior, and conceals a primitive 

 form quite different from that of the garnet, this last bein*' 

 the dodecahedron with rhombic planes. 



Let us now choose for a primitive form the rhomboid 

 represented by figure 27, which differs from the cube in 

 being a little more acute. 



Suppose that the laminae which adhere over all the faces 

 of this rhoiTiboid decrease solely on the angles contiguous 

 to the summits A, O', and that this decrement takes place 

 by two ranges ; then, instead of tuenty-four faces no more 

 than six will be formed ; and it' we conceive them prolono;ed 

 until they meet, they will compose the surface oF a v^ry 

 obtuse rhomboid, which will be ihe secondary form. 



Fig. 28 repre-.ents this latter rhomboid with its nucleus. 

 We there sec that iis summits A, O' are blended with those 

 ot the prin)itivc rliomboid, which are the parlinsr limits of 

 the decrements, aird that each of its laces, suciras Aeoi, 



* Of aiialciirte (i, e. impotent) Haiiy has discovered onlv two speries, thi^ 

 )t. lT<i})<7a(tal and A.lric'poinii; the l.itler is tliCHv/'^W jx'j/i/'i Of MV-riier, ar.d 

 cubic zeolite of Brochaui. — 2ia;w, 



covicspoa.is 



