Mr Haidinger on Sulp}iato4ri~Carhonate of Lead. S8'7 
Rhomhoidal, which he describes * as presenting the form of 
a regular six-sided prism, or also that of an acute rhombohe- 
dron of 70° SS' (the plane angles being given = 60° and 120°), 
diversely modified by planes perpendicular and parallel to its 
axis. He considers the acute rhombohedron as the primitive 
form of the species, Mr Brooke “I", who calls this substance 
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° 80', 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 Phil- 
lips’ Mineralogy, page 842., 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 
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 direc- 
tion intersecting the principal axis. Contrary to the general 
observation, that rhombohedral substances possess only one axis 
of double refraction. Dr Brewster J found that the mineral in 
question exhibited two axes or 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 prismatic 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 sufficient- . 
ly correct result ; but having lately resumed this examination, 
the beautiful specimens in Mr Allan’s collection, and several 
merit of several of the varieties will be found in The Edinburgh Transactions, 
vol. X. part I. 
* Catalogue de la Collection Mineralogique, p. 343* 
Edinburgh Philosophical Journal, vol. iii. p. 118. 
$ Edinburgh Philosophical Journal, vols. iii. p. 138. ; vi. p. 183, ; ix, p. 367. 
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