218 MR HAIDINGER on the Forms of Crystallisation 



to this law, and to indicate certain exceptions to its generality ; 

 but as the existence of two polarising axes in a mineral, when- 

 ever it is capable of being observed at all, can always be ascer- 

 tained in the most unequivocal manner, and as several of the cry- 

 stallographic observations date from an early period of the sci- 

 ence, these exceptions are less likely to arise from the want of cor- 

 rectness or generality in the law, than from a want of precision 

 in observing the properties of minerals ; and hence a careful re- 

 examination of them is a subject deserving the particular atten- 

 tion of crystallographers. The Sulphato-tri-Carbonate of Lead 

 from Leadhills, as hitherto described, constitutes one of these 

 exceptions. It is the object of the present paper to remove the 

 difficulties in respect to this species, by a more accurate inves- 

 tigation of its crystalline forms. 



Count BOURNON seems to have been the first author who 

 erected this substance into a separate species, under the name of 

 Plomb Carbonate Rhomboidal, which he describes * as presenting 

 the form of a regular six-sided prism, or that of an acute rhom- 

 bohedron of 70 32' (the plane angles being given 60 and 

 120), variously modified by planes perpendicular and parallel 

 to its axis. He considers the acute rhombohedron as the primi- 

 tive form of the species. Mr BROOKE f, who calls this substance 

 the Sulphato-tri-Carbonate of Lead, likewise states the form of 

 the crystals most commonly occurring, to be a regular six-sided 

 prism, or an acute rhombohedron of 72 30', terminated by a 

 plane perpendicular to its axis ; the latter being parallel to the 

 perfect planes of cleavage. He mentions, besides, a consider- 

 able number of secondary faces, and he has given the drawing 

 of a variety, which contains them, in the third edition of PIIIL- 

 LIPS'S Mineralogy, page 342., where he likewise assumes the 

 acute rhombohedron as the primary form of the mineral. Sup- 

 posing the angles given by Mr BROOKE to be exact, Professor 



* Catalogue de la Collection Mineralogique, p. 343. 

 j- Edinburgh Philosophical Journal, vol. iii- p. 118, 



