182 Mr. W. Phillips on the Forms of Sulphur ct of Bismuth. 



one principal cleavage, of which however I could not ascertjvin 

 ihe position. 



Fi.r. 1. 



M on M' 9P 00' 



M on Ze 134 30 



Fig. 2. 



M on M' 91° 30' 



M on // 134- 30 



h oni\ 137 40 



{2 156 35 



is 162 25 



The second figure represents some crystals from the Cornish 

 specimen, and the measurements obtained from them are an- 

 nexed. The near coincidence between the natural and arti- 

 ficial crystals is remarkable ; and perhaps the small difference 

 of 30', occasioned by the artificial affording M on M' 91 °, while 

 the natural gave 91° 30', may be accounted for by the ex- 

 treme difficulty of separating the natural crystals from the 

 matrix without bending them in some degree, since almost 

 every one was attached to it at both terminations ; and I am 

 inclined to believe 91° to be the more correct measurement, 

 since that of M upon h agrees in both cases: in both, the 

 plane f is too imperfect for measurement. 



The only bright cleavage of the native sulphuret is parallel 

 to the plane h ; but there does exist a cleavage at right angles 

 to it, though difficult of attainment and far from bright: in 

 one instance I obtained an incidence of 90^ by the reflective 

 goniometer. Cleavages also appear to exist parallel to the 

 planes M M', but I did not succeed in deciding the fact by 

 the goniometer. The prisms allow a cleavage sufficiently 

 bright for its use in a direction at right angles to the axis, af- 

 fording P on M or //, 90'=. 



From the preceding circumstances I am induced to consi- 

 der the primary form to be a right rhombic prism of 91° and 

 89°. 



XXXVI. Oullines 



