476 Mr. L. Fletcher's Crystallographic Notes. 



original paper of Scheerer, and assuming that the information 

 contained in the last-mentioned ' Mineralogy ' incorporated all 

 the results of observation up to that date, determined anew 

 the form (2 1 1), and also added a triakisoctahedron (3 3 2), a 

 tetrakishexahedron (3 1 0), and a hexakisoctahedron (6 4 3). 



The prominent forms, according to both Scheerer and vom 

 Rath, are the octahedron (111), the dodecahedron (110), 

 and the icositetrahedron (21 1); while, according to the latter 

 mineralogist, the additional forms (3 3 2), (3 1 0), (6 4 3) are 

 only subordinately developed. The disposition of the faces of 

 this hexakisoctahedron (6 4 3) will be more easily imagined, if 

 it be remarked that they cut off the edges of intersection of 

 the octahedron with the tetrakishexahedron (3 1 0). With 

 regard to the faces of the triakisoctahedron, vom Rath observes 

 that they are small and unsuited for measurement, but that in 

 one case he had been able to measure the angle made with the 

 adjacent face of the octahedron as lying between 9^° and 10|°; 

 whence he concludes that the form is not (2 2 1), as given by 

 Miller, which requires an angle of 15° 48', but (3 3 2), for 

 which the calculated angle is 10° 2'. 



As vom Rath considered it unlikely that the subordinate 

 triakisoctahedron present on some crystals should be (2 2 1) 

 and on others (3 3 2), he suggests that the symbol given by 

 Miller is a mistaken one, and appears to think that the form 

 (2 21) may have been determined by inspection and not by 

 measurement. It seems, however, much more probable that 

 the information given by Miller is intended to be merely 

 a statement of the results of Scheerer, and not of a later 

 examination by Miller himself, who could not have failed 

 to remark the existence of the icositetrahedron (2 1 1), which 

 is so characteristic that it was present on every crystal ex- 

 amined by Scheerer, and in fact can be distinguished on 

 every specimen in the collection of the British Museum. It 

 seems fairly evident, then, that Miller's (2 21) was a simple 

 error introduced in the copying of Scheerer's results, and that 

 the angles were afterwards calculated without this error in the 

 translation of the symbol from the German notation being 

 remarked. If this hypothesis be correct, (2 2 1) must be 

 expelled from the list of observed forms. 



The next mention of Skutterudite is to be found in an ex- 

 tremely interesting and important paper by Schrauf and Dana 

 on the thermoelectric properties of mineral-varieties*, where 

 they remark that, although the mineral undoubtedly crystal- 

 lises in the cubic system, and has never been observed to 



* <( XJeber die thermoelektrische Eigenschaf ten von Mineral- Varietaten," 

 Sitz.-Ber. Ak. Wteii, vol. hjx. p. lo3 (1874). 



