70 Scientific Notices — Mineralogy. [Jan. 



bases. This salt dissolves without undergoing- decomposition 

 in dilute liquid ammonia, forming with it a brown coloured solu- 

 tion : in water it dissolves only partially, azote being at the 

 same time disengaged, and hyperoxide ot" cobalt precipitated. 

 Exposed to the air, it is rapidly decomposed, and becomes dull 

 and red coloured : it seems probable that the decomposition is 

 principally occasioned by the absorption of carbonic acid. — 

 Leopold Gmelin. 



Mineralogy. 



3. Composition of Garnet. 



When writing the short notice on garnet contained in our num- 

 ber for Nov. last (vol. viii. p. 388), we were not aware that a 

 systematic examination of this mineral had been aheady under- 

 taken and accomplished. Having since received the entire 

 volume of the Swedish Transactions for 1823, we find in it a 

 memoir by Wachtmeister, containing a description and analysis 

 of 13 varieties of Garnet, all from different localities and geolo- 

 gical positions. With only one exception they all proved to be 

 constituted in conformity with the formula which we gave in 

 our notice, namely, an atom of a silicate of a base containing 

 three atoms of oxygen (as alumina, peroxide of iron), combined 

 with an atom of a silica of a base containing two atoms of oxy- 

 gen (as lime, magnesia, protoxide of iron, protoxide of manga- 

 nese). Whenever a genus becomes so diversified as is the case 

 with garnet, it is of the utmost consequence, in a mineralogical 

 point of view, to investigate the connexion which subsists 

 between the chemical composition of each variety, and its 

 external and physical characters, such as its specific gravity, 

 hardness, colour, transparency. We have, therefore, arranged 

 the most important of Wachtmeister's results in the form of a 

 synoptical table ; by means of Vv'hich the mutual relations 

 between the principal characters of each variety will be made at 

 once apparent to the reader. In a geological point of view, it is 

 of no less importance to trace the degree of similarity which 

 subsists between the composition of a simple mineral and its 

 matrix ; and as the garnet appears to surpass almost every other 

 class of minerals in the remarkable extent to which its composi- 

 tion is influenced by that of the substance in which it exists 

 imbedded, or upon which it rests, we have allotted a column 

 to the matrix of each variety, or, where that has not been men- 

 tioned, to the minerals with which it is found associated. 



