m the Optical System of Mineralogy . 369 
& mere shadow. — When he has pledged his character as a 
crystallographer, that the sulphato-tri-carbonate of lead has an 
acute rhomboid for its primitive form, it will then be time to 
consider whether or not such a result is hostile to the optical law. 
Let us suppose, however, that this result is obtained, and 
that all the cry stallograph ers of Europe concur in opinion that 
the primitive form of that mineral 44 is a solid, Contained within 
six equal rhombic planes, and having tw'o of its solid angles; 
and only two, composed each of three equal plane angles, ” 
Then, because the mineral which crystallises in the form of this 
solid has two axes of double refraction, I maintain that its pri- 
mitive form is the oblique rhombic prism , which forms the 
limiting solid between the two series of oblique rhombic prisms, 
the side of whose base or summit plane is either less or greater 
than the sides of any of its other planes. Upon the same prin- 
ciple, I have already maintained, and some of the first crystal- 
lographers have agreed with me in opinion, that Boracite * 
iS not a Cube but a Rhomboid of 90®. 
Here, then, lies the beauty, and the power of the optical sys- 
tem. It not orily determines the class of primitive fortes, but 
it determines, what Crystallography is incapable of doing, those 
limiting solids which form the nodes of every series of geome- 
trical bodies. If we take a crystal, for exafripJe, bounded by 
six equal square faces, the crystallographer will content himself 
with calling it a cube ; but the optical mineralogist will only call 
it a cube when it lias no double refraction. He will maintain it 
to be a rhombs if it has a single axis of double r fraction's coin- 
cident with one of its diagonals ; and lie will consider it as a 
right prism with a, square base's if it has a single axis of double 
refractions perpen dicular to any two of its faces. T he position of 
the axis of double refraction shews, that the mechanical condi- 
tion of the parts of the crystal are related to that line in which 
it is invariably found in every rhomboid, arid in every prism 
With a square base. 
I may now r be permitted to add, that while the optical sy stem 
iri this respect leaves the ordinary resources of crystallography 
far behind it ; it also enjoys the peculiar property of determi- 
ning all structures of composition to which the crystallographer 
* See Edin. Phil. Journal , Voh V. p, 217. 
VOL. IX. NO, 18. OCT. 1823. A a 
