98 Mr. J. F. W, Herschel on the [Feb. 



and almost solid coagulum. Again, if to the coagulum so 

 formed, a quantity equally inappreciable of the original ferrugi- 

 nous solution be added, it gradually liquefies, and after some 

 time is completely redissolved (forming no inapt representation 

 of the celebrated imposture of St. Januarius's blood).* 



A similar change is produced by an increase of temperature. 

 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.f 



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 application 

 of this one of their properties, to an important object in analysis. 

 The principle here developed furnishes a ready method of detect- 

 ing the minutest quantities of other metals in union with iron, 

 and, therefore, cannot but prove of important service in various- 

 cases where this metal constitutes the chief ingredient in the 

 substance examined, as in meteoric iron, the various natural 

 oxides of this metal, 8cc. 8cc. I will exemplify this in one or 

 two instances. 



36*00 grains of meteoric iron (furnished me by the kindness 

 of Dr. Wollaston) were dissolved in dilute nitro-sulphuric acid, 

 leaving behind a minute quantity of a brilliant black powder, 

 -which, however, dissolved by digestion in nitromuriatic acid, 

 and appeared only to contain an excess of nickel. The solu- 

 tions were mixed, and being neutralized at a boiling tempera- 



• The phacnomenon described in the text appears to me to differ from ordinary preci- 

 pitations and sohitions, in the small proportion between the precipitant and the precipi- 

 tate, the solvent and the matter dissolved. I can call to mind but one instance 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 substances. The action 

 seems to be propagated from particle to particle. M'hether the superabundant oxide of 

 iron be retained in solution in a state at all analogous to that of the oxygen in Thenard'« 

 €Kperiments, might possibly deserve consideration. 



t 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 temperature, but decomposed 

 by heat. That this is not the true explanation is already shown, and I have considerable 

 doubt of tlie existence of a percarbonate of iron at tnii/ tatipcraturc. 



The most elegant mode of exhibiting tlic experiment is, perhaps, the following; — 

 Having rendered a solution of protosulphate of iron rigorously neutral, by agitation with 

 carbonate of lime and filtration, dissolve in it a small quantity of chlorate of potash (a 

 salt perfectly neutral). The solution when raised to ebullition is peroxidized, a quantity 

 of subsulphate precipitates, and the supernatant liquid is found decidedly, and even 

 strongly add. 



