146 Mr Haidinger on the Sulpfiato-tri-carhcmate of Lead. 



lead, which I am confident will lead to such a thorough ex- 

 amination of this substance, as cannot but remove every doubt 

 or difficulty still prevailing among many mineralogists as to 

 the perfect establishment of the species in all its physical pro- 

 perties. 



During an optical examination lately commenced, Mr Brooke 

 was surprised to find that the two first crystals of the sulpha- 

 to-tri-carbonate of lead, the first substance he examined, had. 

 only one axis of double refraction. * They had the form of 

 rhombohedrons with truncated apices. One of them was yel- 

 low, the other green. He found this to be the case in all those 

 varieties whose forms appeared to be rhombohedrons with 

 truncated apices, also in some green hexagonal prisms, and 

 in a yellow hexagonal prism terminated by trihedral pyramids. 

 On the contrary, he found the flat crystals, some yellowish, 

 or colourless, or greenish, to have two axes inclining to each 

 other at small angles, for the accurate admeasurement of which 

 he was getting constructed a particular instrument. He says 

 that he has not yet sufficiently examined the crystalline forms 

 of the biaxal crystals ; but that he suspects they will, if not 

 rhomboids, turn out to be right rhombic prisms. 



Mr Brooke proposes also to examine chemically these two 

 sets of crystals, which, on the supposition of their forms being 

 connected partly with the right rhombic prism, and partly 

 with the rhombohedron, provided also that they contain the 

 same ingredients, would be in a similar relation to each other, 

 as arragonite and calcareous spar. 



* I have examined some of these crystals sent to Mr Haidinger, and 

 find that they have one negative axis of double refraction coincident with 

 the axis of the rhomb, and without any trace of composition. The optical 

 law of primitive forms thus derives an additional support from this ob- 

 servation. — D. B. 



