of the Suiphato-tri-Carbonate of Lead, 219 



MOHS also considered it as rhombohedral, and arranged it in the 

 genus Lead-baryte, under the denomination of the Axotomous 

 Lead-baryte, being most distinctly cleavable in a single direction 

 intersecting the principal axis. Contrary to the general obser- 

 vation, that rhombohedral substances possess only one axis of no 

 polarisation, Dr BREWSTER * found that the mineral in question 

 exhibited two systems of coloured rings, more distant from each 

 other than those of Carbonate of Lead ; and from the existence 

 of the two axes, he inferred that its forms belonged to the pris- 

 matic system. He also remarked, that many crystals contain films 

 oppositely crystallised, as is the case in Arragonite. 



I had at various times attempted to examine the forms of 

 Axotomous Lead-baryte, without, however, attaining a sufficiently 

 correct result ; but having lately resumed this examination, 

 the beautiful specimens in Mr ALLAN'S collection, and several 

 others, equally interesting, with which I had been favoured by 

 Dr BREWSTER, Mr IRVING, and Mr T. DOWLER, have enabled me 

 to ascertain the forms of this species with a considerable degree of 

 accuracy. The results of this examination are very remarkable. 

 They exclude the rhombohedron and the regular six-sided prism 

 from the range of forms, which the individuals of the species 

 may assume, and thus perfectly confirm the inference drawn 

 by Dr BREWSTER from his optical observations, while theyrtare at 

 variance with the crystallographic statements both of Count 

 BOURNON and of Mr BROOKE. 



The crystals of Axotomous Lead-baryte have been described 

 as regular six-sided prisms, having angles of 120, and termi- 

 nated by planes perpendicular to their axis. Upon examining 

 them, however, by the reflective goniometer, I found that neither 

 of these statements was correct, but that the six-sided prism is 

 a combination of three different simple forms, , b, and c, Fig. 1 . 

 Plate IX., whose angles of intersection differ more or less from 



* Edinburgh Philosophical Journal, vols. iii. p. 188. ; vi. p. 183. ; ix. p. 367; 



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