587 
Cast Iron^ Steel, and Malleable Iron. 
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 care 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 
consider, 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 carburet 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 spathose iron 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 ail 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. 
