Genth.] ^2 [Oct. 2, 



and not in accordance with the present views of chemical combination, 

 a new analysis was very desirable. 



Mr. Clarence S. Bement, who has in his magnificent cabinet a fine 

 cleavage mass of about four inches in diameter, has kindly presented me 

 with the material for this purpose. 



The specimen received is of a dark steel-gray color and shows the most 

 perfect lamination and cleavage. Between the laminae could be observed 

 a greenish and yellowish coating, which when magnified 100 diameters 

 showed a crystalline structure. I also observed, under the microscope, a 

 very minute quantity of yellowish-white globular aggregations. These 

 coatings are the product of a partial oxidation of the mineral and were 

 readily removed by dilute hydrochloric acid. They are probably mon- 

 tanite. 



The analysis of the purified material gave results very close to those ot 

 Damour : 



Te = 



Se = 



S = 



Bi = 



100.20 99.71 98.66 



This composition cannot be expressed by a rational formula. There is 

 also a doubt about its crystalline form, which is generally taken as hexago- 

 nal, with an eminently basal cleavage, although crystals, as far as I know, 

 have never been found or examined. 



Similar doubts exist about the form and rational composition of tetrady- 

 mite, a question left open by Groth and others. 



On crystals from Schubkau, in Hungary, Haidinger determined the form 

 as rhombohedral, with perfect basal cleavage. 



I am not aware that since then a crystallographic examination of this 

 mineral has been made. These crystals are mostly dull, distorted and 

 striated and not the best material for measurement. No other locality has 

 furnished specimens in well defined crystals. It is very probable, how- 

 ever, that Haidinger's determination is correct, judging from a pseudo- 

 morph of gold after tetradymite from the Whitehall Mine, Spottsylvania 

 county, Va., in the collection of the University of Pennsylvania, showing 

 forms which appear to be combinations of a rhombohedron with a scale- 

 nohedron and the basal plane. 



This does not exclude, however, that tetradymite may not also occur in 

 rhombic forms and that the characteristic eminent cleavage may not be 

 basal, but brachydiagonal, as in bismuthinite, stibnite and orpiment, 

 which have an analogous composition. 



It was Gustav Rose who first suggested that tetradymite was bismuth 

 with a variable quantity of isomorphous tellurium, and this opinion has 



