depending on their Internal Changes. 281 



fundamental rhombohedrons being much enlarged. Some of 

 them have their broad faces in the direction of the faces of the 

 octahedron ; and in some of the octahedral groups this cir- 

 cumstance has produced a kind of raised reticulated appear- 

 ance on the adjoining faces of the original octahedron, which 

 the newly formed crystals intersect, and project beyond them. 

 The changes which affect the sparry iron deserve our parti- 

 cular notice, as they are not only highly interesting in them- 

 selves, but have been well attended to at all those places where 

 this species forms the predominant ore of iron. The charac- 

 teristic chemical ingredient of it is the carbonate of iron, Fe C 2 , 

 in which the protoxide of iron and the carbonic acid are in the 

 ratio of 61.47 and 38.53. It contains occasionally an admix- 

 ture of the carbonates of lime, magnesia, and manganese. 

 The colour of the original varieties is usually a pale yellow, 

 inclining to gray ; the lustre and transparency are consider- 

 able. When left exposed to the action of the atmosphere, the 

 surface soon assumes a brown tint, which by degrees pene- 

 trates deeper into the substance of the crystals. Some lustre 

 even then remains, and cleavage is still observable. Speci- 

 mens bounded by fissures on all sides, or broken out of a solid 

 mass, when examined in this stage of their decomposition, 

 often still contains a nucleus of the yellowish-gray undecom- 

 posed substance. When the decomposition has arrived at its 

 end, every trace of cleavage has disappeared, the fracture of 

 perfectly well pronounced crystalline shapes is uneven, or 

 earthy, and the colour a dark brown, which is likewise visible 

 in its streak. The substance now consists of a compact variety 

 of the hydrate of peroxide of iron, whose chemical composition 

 is expressed in the formula 2 Fe -{- 3 Aq. and which contains 

 14.7 per cent of water. One atom of the carbon contained 

 in the original compound will therefore go away in the state 

 of carbonic acid, while the other must be transformed into 

 oxide of carbon, in order to convert the protoxide of iron into 

 a peroxide. The change in those masses has taken place so 

 insensibly, that the action of the power of crystallization was 

 prevented, and the interior presents a pretty uniform texture ; 

 but, at the same time, some particles of the hydrate of iron 

 commonly also follow their own innate attraction, and form 



