62 Dr, Thomson on some New Minerals. 



Its specific gravity is about 254:1, which agrees with that of com- 

 mon felspar ; and its hardness is about the same as that of felspar. 

 The texture is compact, or at least not sensibly foliated, and I have 



101-35 



7 (A1S») + Mg. S'+ KS'. 



So that it differs from felspar by one half of the potash being replaced 

 by magnesia. 



2. Perthite. — The next mineral 1 have to notice, I distinguish by 

 the name of Perthite. It was sent me by Mr. Wilson, a surgeon in 

 Perth, a township of Upper Canada. Hence the name by which I 

 have distinguished it. It is very much connected with felspar in 

 appearance, and was sent as a variety of that mineral. 



The colour of the specimen sent me is white. It consists of a mass 

 of crystals so united together as to form a kind of tesselated pavement. 

 The crystals are obviously four-sided prisms, apparently rectangular, 

 but not susceptible of measurement, because they cannot be isolated. 



The lustre is vitreous ; the hardness is rather less than that of felspar ; 

 but the specific gravity, which is 2*586, is identical with some of the 

 varieties of that mineral. 



The constituents were found to bo 



99-625 



From this analysis, it is evident that it differs essentially from 

 felspar. The quantity of silica is much greater, and the potash is 

 entirely replaced by magnesia. Its constitution may be represented 

 by the formula 6 (AIS*) + 5 (Mg. S*). It is a quatersilicate, while 

 felspar is a tersilicate. Could it be procured in sufficient quantity, it 

 would be an excellent material for the manufacture of porcelain. 



3. PerisCerite.* — The next mineral which I have to mention, was 



* From Ti{«rT£5» a pigeon, the colours resembling a pigeon's neck. 



