separation of iron from other metals. 297 
A similar change is produced by an increase of tempera- 
ture. If we heat a solution exactly neutralized as above 
described, it speedily grows turbid, deposits its ferruginous 
contents in abundance, and at the same time acquires a very 
decided acid reaction. The acid so developed holds in solution 
a portion of oxide, but if the neutralization be performed 
afresh while hot, this separates entirely, and the liquid after 
filtration has no more action on gallic acid, ferrocyanate, or 
sulphocyanate of potash, than so much distilled water.* 
It is not my object in this paper to enter into any minute 
detail of the nature of the persalts of iron, a subject not nearly 
exhausted, and which want of leisure alone has prevented 
my entering upon, but merely to point out the practical ap- 
plication of this one of their properties, to an important 
precipitate, the solvent and the matter dissolved. I can call to mind but one in- 
stance of so small a quantity of matter operating a chemical change on so large a 
mass, viz : the decomposition of oxygenated water by fibrin and other animal sub- 
stances. The action seems to be propagated from particle to particle. Whether 
the superabundant oxide of iron be retained in solution in a state at all analogous 
to that of the oxygen in Thenard’s experiments, might possibly deserve con- 
sideration. 
* It was in 1815, in the analysis of a specimen of the gold ore of Bakebanya, 
given me for that purpose by Dr. Clarke, that I first remarked the separation of 
oxide of iron from a clear neutral solution by mere elevation of temperature, and 
attributed it to the presence of an oxycarbonate capable of subsisting in a low tem- 
perature, but decomposed by heat. That this is not the true explanation is already 
shown, and I have considerable doubt of the existence of a percarbonate of iron at 
any temperature. 
The most elegant mode of exhibiting the experiment is perhaps the following : 
having rendered a solution of proto-sulphate of iron rigorously neutral, by agi- 
tation with carbonate of lime and filtration, dissolve in it a small quantity of chlo- 
rate of potash (a salt perfectly neutral). The solution when raised to ebullition is 
peroxidized, a quantity of sub-sulphate precipitates, and the supernatant liquid is 
found decidedly, and even strongly acid. 
