of Mineral Species. 91 



rites, having the same mixture, are also subject to the same 

 change : the sulphur goes away, and the iron takes up oxygen 

 and water ; the decomposition proceeds from the surface. We 

 often see crystals covered on the surface with a brown tarnish, 

 and this is the first stage of the change. There are specimens 

 with a thin coat of the hydrate of iron ; there are others consist- 

 ing almost entirely of the latter, with only a nucleus left of the 

 original bisulphuret of iron. Such are found at Wochein in Car- 

 niola, where this hydrate of peroxide of iron, produced from the 

 decomposition of the bisulphuret, occurs in such abundance and 

 pureness, that it is melted as a very valuable ore of iron. The 

 iron extracted from it is particularly remarkable for its softness. 



V. Changes in Minerals containing Lead. 



The mineral called Native Minium is probably, in every in- 

 stance in which it has yet been observed, the product of decom- 

 position of some other substance containing lead. Such is the 

 variety which M. BERGEMANN of Berlin found in the lead mines 

 of Kail, in the Eiffel in Germany, where the ore, chiefly the sul- 

 phuret and carbonate of lead, is dug out in irregular masses, 

 from the loose earth, to the inconsiderable depth of a few fa- 

 thoms. To him I have been indebted for several distinct crys- 

 tals, possessing the regular forms of the di-prismatic lead-baryte, 

 not only in regard to the simple prisms and pyramids of which 

 the combinations consist, and the striae on the surface of some 

 of them, but also in regard to the identical mode of being joined 

 in twin-crystals. The beautiful red colour, which, in these com- 

 pact masses, much more nearly approaches the colour of vermi- 

 lion, than in the best varieties of the usual minium in the state 

 of powder, and the apparent homogeneity of the mass in the 



M 2 



