Intelligence and Miscellaneous Articles. 159 



But If this were the cause which unites all these substances in 

 this group, their forms ought to arrange themselves uniformly around 

 that of the cube, regarded as a rhombohedron of 90°. This is not 

 the case. The angle of the primitive rhombohedron for all these 

 twenty-six bodies, varies between 83° and 87° 40' ; its average value 

 is about 85° 30'. From 88° to 94°, or between the most extended 

 limits comprising the angle of 90°, there are only ten substances, 

 amongst which only one, the chloroplatinate of rethylamine, is re- 

 lated in its nature to the compounds belonging to the cubic system. 

 The author therefore considers that this hypothesis must be aban- 

 doned, but does not propose another. 



In the square prismatic system there is also a tolerably numerous 

 group of substances of which the forms may be derived from square 

 octahedra very near to the regular octahedron. This group, however, 

 presents less interest than the preceding one. Of thirty substances 

 of which it is composed, there are only eight or ten which offer, in 

 their constitution, some relations to the cubic system ; these are, — 

 chlorate and bromate of silver, iodate of ammonia, iodide and cyanide 

 of mercury, braunite, and iodide of tetramethylammonium. On the 

 other hand, 'the forms of these substances group themselves with 

 tolerable uniformity around that of the regular octahedron, which 

 they approach closely, so that there would be no difficulty in re- 

 garding them as really isomorphous with the bodies to which they 

 approach in constitution, and which crystallize in the regular 

 system. — Comptes Rendus, Oct. 26, 1857, p. 650. 



ON THE COMPOSITION OF EPIDOTES, VESUVIANS AND GARNETS. 

 BY R. HERMANN. 



1. Epidotes. — The old opinion that these minerals are similar in 

 composition, so that their proportions of oxygen are RO : R^O^ : SiO- 

 = 1:2:3 (which composition is still admitted by Rammelsberg), is 

 rejected by the author. Nor does Scherer's opinion, tliat in epi- 

 dotes water : RO and the alumina and oxide of iron : SiO' are 

 polymerically isomorjjhous, so that their formula is (RO)^ (SiO^)*, 

 agree with the author's analyses of epidotes in which he determined 

 tlie protoxide of iron. 



Hermann rather supposes that in epidotes 3 atoms RO may 

 displace 1 atom R^ O^, and that the fundamental mixture of all 

 minerals crystallizing in the form of epidotc consists of the molecule 

 (RO, R'2 O^)'' SiO-. This fundamental molecule may combine with 

 various quantities of diversely composed accessory molecules without 

 injury to its form. 



In the ci)idotes, in the restricted sense, the accessory molecule 

 consists of UO, HO ; in the orthitcs, of HO. 



The general formula of the epidotes in the restricted sense is there- 



