402 Mr. W. Phillips on the Crystalline Form ofSillmaiiite. 



plane of cleavage parallel to one of the diagonals of the prism, 

 it would satisfactorily prove them to be that substance : and 

 the attempt succeeded ; but I found it to be parallel to the 

 shortest diagonal of the prism. 



Again, taking advantage of this cleavage, I obtained it on 

 some of the larger prisms having several lateral planes : it is 

 represented by / on the second of the following figures ; and 

 the measurements accompanying the figures were aiterwards 

 obtained. 



M on M' about 88° 



/ ... 134 



g\ on f ... 145 

 g2on f ... 152 



M 



ffl 



SZ f 



3Z 



M 



From the foregoing circumstances I am led to believe, that 

 a rhombic prism of 88° and 92° may, in default of better evi- 

 dence, be adopted as the primary form of this mineral : the 

 planes of this prism are brighter than the other planes. The 

 terminations of the prism are, according to Bowen, oblique to 

 the axis ; but 1 have not succeeded in my attempts to find 

 any indications of cleavage in that direction, nor at right an- 

 gles to the axis. The crystals, indeed, are often separable 

 with ease nearly in that direction, owing apparently to natural 

 fissures across it; but the surfaces produced by the fracture are 

 neither even nor brilliant, nor at a constant angle with the 

 axis. Besides the brilliant cleavage already mentioned, im- 

 perfect indications of another at right angles to it may some- 

 times be observed. 



From the close agreement in the analysis of this mineral 

 with that of kyanite, and the measurements adopted by M. 

 Haidinger, as may be assumed from Bowen, which also agree 

 very nearly with those of that mineral, he is of opinion that 

 sillimanite is probably a variety of kyanite; an opinion which 

 it is probable will at least receive a revision, on taking into 

 consideration the circumstances above detailed relating to 

 form, measurement and cleavage, in all which the two minerals 

 seem to disagree very decidedly. 



LXXIX. Re- 



