1823.] Proceedings qf Philosophical Societies* 161 



chemistry and crystallography concur so perfectly as they do in 

 this instance, in determining the species, to which a mineral 

 belongs, it will be difficult to admit a variation of optical charac- 

 ter, as a sufficient ground to alter that determination. 



" A paragraph published by Dr. Brewsteir in the sixth volume 

 of the same Journal, p. 183, relative to the crystalline form of the 

 sulphato-tri-carbonate of lead, furnishes an additional motive to 

 beheve that the connexion between the optical characters of 

 minerals and their crystalline forms is not yet sufficiently under- 

 stood. 



*' Dr. Brewster admits what I believe is not liable to question, 

 that ^ the crystals of this substance are acute rhomboids.^ But he 

 adds, ' Upon examining their optical structure, I find that they 

 have two axes of double refraction, the principal one of whica 

 is coincident with the axis of the rhomb. The sulphato-tri-car- 

 bonate, therefore, Cfl???io^ have the acute rhomboid for its primitive 

 form, but must belong to the prismatic system of MohsJ 



" But it appears from the * Outline of Prof. Moh's new Sys- 

 tem of Crystallography,' published in the third volume of the 

 same Journal, that a rhomboid cannot belong to his prismatic 

 system. For it is stated in p. 173, that * The rhomboid, and the 

 four-sided oblique-based pyramid,' (the fundamental form of the 

 prismatic system) ' are forms which cannot by any means be 

 derived from each other ; the {two) groups of simple forms, as well 

 as their combinations, must each be always distinct from {the) other* 

 '" If therefore in the hands of Dr. Brewster,'' Mr. Brooke 

 justly concludes, " the use of optical characters cannot at pre- 

 sent be rehed upon for the determination of a mineral species, it 

 may be doubted whether they can be successfully employed by 

 less accurate and less intelUgent observers." — ?. 



Article XI. 



Proceedings of Philosophical Societies, 



LINNEAN SOCIETY. 



May 24. — This being the anniversary of the Society^ the 

 election of the Council and Officers for the ensuing year took 

 place ; when the following gentlemen were chosen. 



Council. — James Ebenezer Bicheno, Esq.; Edward Rudge, 

 Esq. ; Joseph Sabine, Esq. ; Robert Brown, Esq. ; John George 



planes which appear as the summits of some of these prisms, being only the lateral 

 jduncs of very short and otherwise disproportioned crystals ; so that a line passing 

 through these, in the direction of their greatest length, would in fact be perpendicular 

 to the axis of the primary form. Sections perpendicular to the axes of these apparently 

 similar prisms would certainly present very different optical phenomena. But it is not 

 probable that the practised eye of Dr. Brewster should have been misled by their appa- 

 rent similarity ; and the differences he has observed will still remaia to be explained." 



