278 Mr Haidinger on the Regular Composition 



green tints which appear in the finest pearls are analogous to the 

 pink and green masses which I have particularly examined in 

 mother of pearl, and have described in the Phil. Trans, for 1815. 

 The unequal mottled surface of the pearl, which resembles that 

 of very fine ground glass, presents a distinct reflexion from the 

 outer surface, and contributes to disperse and mix the various 

 reflexions which come from the interior of the pearl. 



Art. XXIV. — On the Regular Composition of Crystallized 

 Bodies. By W. Haidinger, Esq. F. R. S. E. Commu- 

 nicated by the Author. Illustrated by Plate VI. — (Conti- 

 nued from Vol. iii. p. 69.) y I \%Q 



IV. — Prismatic System. 



1 he individuals of those species which belong to the prisma- 

 tic system, very frequently present themselves in groupes, where 

 two or more of them are joined parallel to one of their faces of 

 crystallization. Generally this is a face of a rhombic prism, 

 whose axis is either parallel or perpendicular to the axis of the 

 fundamental form. There are a very few cases, however, in 

 which two individuals are joined in a face parallel to one of the 

 diagonals of the base of the fundamental pyramid, and this is 

 possible only upon the supposition, that the secondary faces 

 possess a peculiar hemi-prismatic character; or in which the 

 junction takes place parallel to a face of some scalene four- 

 sided pyramid of finite dimensions. These will be considered 

 more at large hereafter. 



Among the former there is a class of regular compositions, 

 where the junction of two individuals takes place parallel to 

 the faces of prisms whose angles are near 1 20°. The result 

 of it, particularly when three individuals are united, has some- 

 times the appearance of a form belonging to the rhombohedral 

 system, in as much as there is a kind of symmetry established in 

 reference to six faces supposed to meet at angles of exactly 120°. 

 Such are the compound crystals of arragonite, of the di-pris- 

 matic lead-baryte, of strontianite, of witherite, of chrysoberyl, 

 of prismatic copper-glance, of sulphate of potash, and others. 

 Most of them, in the infancy of crystallographic inquiry, were 



