Crystallization in the Mineral Kingdom. 135 



It may happen, and I think it is probable, that pentago- 

 nal dodecahedrons may exist in other species, perhaps in Gers- 

 dorffite, and I fully expect it will be found in Hauerite, which, 

 however, does not belong to that genus. In this case an espe- 

 cial primary form does not exist. 



If iron pyrites and cobaltine could be rendered transparent, 

 they would be found to have one optical axis that coincides with 

 their hexagonal axis. 



I am here reminded that Sir David Brewster recognized bo- 

 racite forty-one years ago, as having one optical axis which co- 

 incided with a hexagonal one. 



It appears very likely that the form of boracite, which has up 

 to this time been regarded as a trigonal dodecahedron, may not 

 be a simple form, but a combination of figures with one axis. 

 I stated at the above-mentioned meeting of the 17th of January, 

 that this was my belief; and the measurements which have been 

 made since that time on some pretty clear crystals, have proved 

 that I was right. 



At first the cube, the pentagonal dodecahedron, and the tetra- 

 hedron gave their proper angles ; but I afterwards found that 

 though three of the planes on the three-edged angles gave the 

 proper inclination towards the hexahedral planes for the trigonal 



dodecahedron, and exactly represented the formula ~, the 



fourth differed materially. 



The first three gave the angle 144° 44', reckoned according 



I j 



to the formula **~; it ought to have been 144° 44' 8"; but the 

 is 



fourth gave 144° 17', a difference of 27 minutes : the crystals 

 presented no difficulty to the measurer. 



These figures, therefore, when placed upright on their hexa- 

 gonal axis, resolve themselves into an acute hemimorphous sca- 

 lenohedron, and into an obtuse hemimorphous rhombohedron, 

 which we will designate as R ; this last form is again more ob- 



xj 

 tuse than if it had been deduced from the formula ^-. The 



figures of the hemimorphous scalenohedron, and of the trigonal 

 prism (the last reminds one of the same form in tourmaline), 

 are placed in such a manner round the three hexagonal poles as 

 the formula requires, and form A ,- *f °f the planes belonging 

 to a deltoid icositetrahedron. The hemimorphous R, on the 

 other hand, gives A=^ of the planes of another deltoid icosite- 

 trahedron. The figures must in this case be held in the same 

 way as they were before examined ; and the hexahedron or the 

 rhombohedron of the rhombic dodecahedron can now be taken as 



