3Ir. L. Fletcher's Crystallogmphio Notes. 475 



Ten years later Scheerer* met with a mineral which ex- 

 amination led him to regard as identical with that previously- 

 described by Breithaupt. A quantitative analysis, afterwards 

 confirmed by Wohlerf, gave the following result: — • 



Arsenic 77*84 



Cobalt 20-01 



Sulphur , 0-69 



Iron 1*51 



Copper traces 



100-05 



a composition expressed by the formula CoAs 3 . 



The description given by Scheerer agrees very closely with 

 that of Breithaupt ; but not only did he find the mineral in 

 its massive condition, but also as isolated crystals. These had 

 the form of octahedra, modified by the cube dodecahedron 

 and an icositetrahedron: the inclination of the faces of the 

 latter form to the adjacent face of the octahedron, as measured, 

 was 19° 27', proving clearly that the icositetrahedron was 

 (2 1 1), for which the calculated angle is 19° 28'; the faces of 

 this particular icositetrahedron truncate the edges of the do- 

 decahedron. He further observes that this form (2 1 1) was 

 present on every crystal he had examined. Scheerer notices 

 also the interesting fact that the crystals are often found im- 

 planted upon crystals of cobaltite, and states that he had not 

 been able to trace any regularity in the relative position of the 

 crystals of the two species. As the term Tesseralkies might 

 be taken to imply that the mineral crystallises in the form 

 of cubes, he suggests Arsenikkobaltkies as a more appropriate 

 name. 



In Haidinger's ' Handbuch der bestimmenden Mineralogie ' 

 (1845) the mineral appears under still another title — Skut- 

 terudite, suggestive of the locality where the crystals are 

 found. 



In Brook and Miller's edition of ' An Elementary Introduc- 

 tion to Mineralogy by the late William Phillips,' published in 

 1852, the forms presented by Skutterudite are given as (1 0), 

 (1 1 0), (1 1 1), and (2 2 1), and the angles according with 

 this description are calculated. It will be observed that the 

 form (2 1 1) described by Scheerer is not mentioned, but that 

 a new form, (2 2 1), a triakisoctahedron, i^ substituted. In 

 1862 vom Kath|, apparently not having referred to the 



* "Ueber zwei norwegische Kobalterze Yon den Skutteruder Gruben," 

 Pogg. Ann. vol. xlii. p. 553 (1837). 

 t Pogg. Ann. vol. liii. p. 591. 

 | "Neue Flacken am Tesseralkies," Pogg. Ann. vol. cxv. p. 480. 



