104 PHYSIOGRAPHY. CLASS u. 



HAUY with the species of rhombohedral Lime-haloide, but 

 is now exhibited as an appendix to Fer oxyde, the prismatic 

 Iron-ore of the system followed in the present work. 

 Some varieties, containing from 0-005 to 0-025 of carbonate 

 of lime, are considered by Professor HAUSMANN in con- 

 nexion with rhombohedral Lime-haloide, while others, 

 without that admixture, form a particular substance, Spltcc- 

 rosiderite, which name refers to the botryoidal and globular 

 shapes, and the curved faces of cleavage. An accurate exa- 

 mination of all the physical properties shews the dif- 

 ficulties connected with the one and the other of these 

 methods of considering the matter, and points out the way, 

 upon which we arrive with perfect security at a correct de- 

 termination of all the varieties which the species contains. 

 Decomposed varieties of the present species have been con- 

 sidered as belonging to prismatic Iron-ore. This, however, 

 is contrary to the principles of Natural History. 



2. Two varieties of this species, 1. the Sphaerosiderite, 

 and 2. a cleavable variety from Neudorf in the Hartz, 

 have yielded to KLAPROTH, 



Protoxide of Iron 1., 63-75 2., 57-50. 

 Carbonic Acid 34-00 36-00. 



Oxide of Manganese 075 3-30. 



Lime 0-00 1-25. 



Magnesia 0-52 0-00. 



The chemical formula of brachytypous Parachrose-baryte 

 is Fe C 2 , which expresses the ratio of protoxide of iron 

 to carbonic acid = 61-47 : 38-53. Before the blowpipe it 

 becomes black, and acts upon the magnetic needle, but does 

 not melt. It colours glass of borax green. It is difficultly 

 soluble, and effervesces but little in nitric acid, particularly 

 if not reduced to powder. On being exposed to the air it 

 is gradually decomposed ; first the colour of the surface be- 

 comes brown or black, from which change of colour the 

 name of the genus has been derived ; afterwards also the 

 streak is changed into red or brown, hardness and specific 

 gravity are diminished, and even the chemical constitution 

 is altered, the whole being converted into hydrate of iron. 



