MINERALOGICAL ARRANGEMENT. 447 



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 

 character, as a sufficient ground to alter that deter- 

 mination. 



A paragraph published by Dr. Brewster in the 6th 

 volume of the same Journal, p. 183, relative to the 

 crystalline form of the sulphato-tri-carbonate of lead, 

 furnishes an additional motive to believe that the 

 connection between the optical characters of minerals 

 and their crystalline forms, is not yet sufficiently 

 understood. 



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 

 " which is coincident with the axis of the rhomb. 

 " The sulphato-tri-carbonate, therefore, cannot have 

 " the acute rhomboid for its primitive form, but must 

 " belong to the prismatic system of Mohs" 



But it appears from the " Outline of Professor 

 Mohs's new system of Crystallography," published 

 in vol. 3 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 tlje pris- 

 matic system) " are forms which cannot by any means 

 " be derived from each other / the (two) groups of 



short and otherwise disfiroportioned crystals ; so that a line passing through 

 these, in the direction of their greatest length, would in fact be per- 

 pendicular 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 prac- 

 tised eye of Dr. Brewster should have been misled by their apparent 

 similarity, and the differences he has observed will still remain to be 

 explained. 



