Cast Iron, Sieel, a?id Malleable Iro7i, 587 



acid solution of peroxide of iron, with other metals and silica, 

 the silica invariably falls down with the sulphurets, perhaps 

 in the state of a sulphuret, and remaining, after the treatment 

 of the sulphurets with aqua regia, in such a state as to be in- 

 soluble in all acids except hydrofluoric acid. 



We cannot enough recommend the utmost cai'e in exami- 

 ning the precipitate obtained by sulphuretted hydrogen from 

 solutions of iron, and all the contents of this precipitate ought 

 to be always separated and tried to be procured in their iso- 

 lated state. 



By trying to separate phosphoric acid from iron, by means 

 of alkalies, the phosphoric acid can only be separated entirely 

 from the iron when the mixture shall be kept in a perfect 

 white heat for some time. 



Since writing this article several months ago, some remarks 

 have occurred to me, which may serve to elucidate it. 



According to the experiments in the previous paper, we 

 considei', that the toughness of the black and gray sorts of 

 cast iron is owing to the siliciuret of iron, while their qualities 

 of strength and fusibility are attributable to the cai'buret of 

 aluminum, silicon and iron. I must here observe, that really 

 gray cast iron, used for foundry purposes, never changes its 

 appearance from gray into white without changing its chemi- 

 cal composition ; while the white crystallized cast iron, pro- 

 duced on the continent from spathoseiron ores, at a compara- 

 tively low degree of heat, changes its appearance from white 

 into apparently gray cast iron, according to the degree of slow- 

 ness with which it is cooled. But this ready conversion of 

 white iron into gray iron is only apparent, and the above- 

 mentioned crystallized iron bears, in either of its states, the 

 chemical character of white iron, in which a part of the silicon 

 is replaced by manganese. When apparently converted into 

 gray iron by slow cooling, it has only changed the state of 

 aggregation of its component molecules, and consequently its 

 density ; or, in other words, the molecules of this sort of iron 

 have had time to arrange themselves during its cooling into 

 a more developed crystalline form. This crystalline form may 

 be easily distinguished from the regular foliated form of crystal- 

 lization of real gray cast iron, when viewed under the micro- 

 scope, by the irregularity, smallness, and thickness of its com- 

 ponent leaves or scales, and a stroke of the hammer will 

 invariably restore to the part struck the original white silvery 

 colour peculiar to it. The residuum of both varieties of this 

 iron, after its treatment with hydrochloric acid, have all the 

 characters of the residuum of white iron ; they are brown, in- 

 stead of white or gray, become ignited at a very low degree of 

 heat, and never effervesce with caustic ammonia. 



2 R2 



