224 M. Rose on those Minerals which possess [March, 



same form and constituent parts. Of this agreement he adduces 

 feldspar and emerald as examples ; but there are other minerals, 

 as pyroxene, amphibole, garnet, and mica, which, possessing 

 similar forms, wherever they occur, differ greatly in their com- 

 position. This last mentioned circumstance has occasioned 

 great difficulty in arranging minerals according to their compo- 

 sition. 



It is observed, that M. Mitscherlich has removed much of 

 this difficulty by showing that certain bodies, when containing 

 the same number of atoms, may exchange their places in com- 

 pounds without inducing any alteration of crystalline form ; 

 according to M. Mitscherlich, most oxides which contain two 

 atoms of oxygen are isomorphous ; and although his experiments 

 were made with artificial salts, it is presumed that the same 

 obtains with respect to crystalline minerals. 



To illustrate this position, M. Rose has observed that the 

 analyses of different specimens of pyroxene show, that all mine- 

 rals which have the crystalline form of pyroxene are bisiliciates 

 of the four isomorphous bases, lime, magnesia, protoxide of iron, 

 and protoxide of manganese ; in all these, the oxygen of the 

 base is to that of the silica as 1 to 2. If the reasoning of 

 M. Mitscherlich be applied to the analysis of various substances 

 classed by Haiiy with pyroxene, it will be observed that they 

 a°ree in composition with that substance ; viz. augit, malacolite, 

 sahlite, baikalite, coccolite, alakit, mussite, diopside, and 

 fassaite. 



The following analyses were made by M. Rose in Prof. Ber- 

 zelius's laboratory at Stockholm, and most of them were several 

 times repeated ; the minerals which possess the crystalline form 

 of pyroxene are classed by M. Rose under the following heads : 



1. Pyroxenes with lime and magnesia as bases. 



2. Pyroxenes with lime and protoxide of iron as bases. 



3. Pyroxenes with lime, magnesia, and protoxide of iron, as 

 well as some protoxide of manganese as base. 



4. Pyroxenes with lime and protoxide of manganese as bases. 



1. Pyroxenes with Lime and Magnesia as Bases. 

 These pyroxenes are mostly white, and form the colourless 

 and usually transparent malacolites, which are sometimes, by 

 slight admixtures, yellowish and greenish. The proportion of 

 lime and magnesia is in almost all which have been analysed 

 very constant, both having the same quantity of oxygen, and 

 together half as much as the silica. 



White Malacolite from Orrijervi, in Finland. 



Is white, only greyish when mixed with galena ; occurs in 

 large crystalline masses with distinct cleavage ; semihard, and 

 very translucent at the edges. Mr. Rose's analysis gives 



