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rubbed off, but can only be feparated by fire.Here then the affinity of the mercury for thecopper mult act forcibly in caufing its repara-tion from the nitric acid which held it in folu-tion. In like manner when copper is immerfedin a folution of Elver, the precipitated metal isnot pure filver, but an alloy, containing a veryfmall portion of copper. As this mutual affi-nity exifting between the metals varies accord-ing to the metal employed, this alfo explainswhy iron (for example) precipitates filver fromits folution with much more difficulty thancopper, though iron has a much ftronger affi-nity for oxygen than copper (as is proved bythe precipitation of copper by iron) for theaffinity between filver and copper is muchftronger than between filver and iron.

As then metallic precipitates are in fa£t al-loys (though often with fuch a fmall propor-tion of the precipitant as to be fcarcely fenfi-ble) it is neceflary that the combination of themetals mult firft be made when both are infolution-, for we can conceive of no other me-thod by which two folid metals (copper andiron, for example) fhould be alloyed in a com-mon temperature. The whole procefs there-fore mult be the following, taking the abovemetals as inftances ; a portion of the iron multfirlt be oxydated and diflblve in the acid, and bythis it dil'oxydates and precipitates in the me-tallic form an equivalent portion of the copper,during which time the diflblved iron mult di-vide itfelf into two very unequal portions, ofwhich by far the largeft part remains in folu-tion, but the remainder mult return to the me-tallic Hate to unite with the copper and to beprecipitated along with it. This complicatedaction mult take place even from the firlt, forhow elle can the union of the two metals beaccounted for ? and Hill lefs can it be explainedwithout tliis fuppofition, as the precipitationproceeds, when the furface in contact withevery freffi ftratum of precipitate is no longeriron, but an alloy of copper and iron almoftapproaching to pure copper.

This leads us alfo to coniidcr what is the affi-nity that determines, the place to which theprecipitates attach themfelves. In the firft in-ftance, that is, when a furface of pure iron istxpofed to the cupreous folution, the precipi-tate may be fuppofed to- attach itielf to the ironrather than to fall down loofe in the veflel onaccount of the affinity exifting between ironand copper alloyed with a little iron; but afterthe furface of the iron bar is covered, each

fuceeffivc ftratum of precipitate attaches itfelfnot to the iron (for this is covered) but to the,lurface of the precipitate already formed,that is to fay* to a fubftance of exaftly thefame compofition as itfelf. In this latter cafetherefore, the fame kind of affinity a£i:s aswhen a falitie folution during cryftallizationclepofits its cryftals on the portion of fait al-ready feparated, rather than on a naked fur-face. The adhefion in this inftance, however,is very /light, and readily broken. The pro-portion of the precipitant which enters into thealloy is probably greater in the firft ftratum ofprecipitate than in the fubfequent ones; thuswhen copper is feparated by iron,, the part im-mediately in contatt with the iron is vifiblybrowner than the reft, owing probably to alarger admixture of iron.

It has been mentioned that phofphorus willfeperate fome metals from their folutions inthe reguline ftate. Thefe are particularly gold,,filver, mercury, and copper. If a ftick of cleannewly-melted phofphorus is immerfed in anacidulated folution of fulphat of copper, in a dayor two it becomes beautifully fpangled with littleknobs of bright metallic copper, which graduallyincreafe to a thick cruft, after which if the ftickis put into boiling water, the phofphorus willmelt out, and leave a delicate hollow cylinderof copper. In this cafe however, as in theformer, the precipitate is not pure copper, butan alloy of copper with phofphorus, or a phof-phuret, in which however the quantity of phof-phorus is extremely fmall, and the folutioncontains phofphoric acid. When phofphorusis introduced into a folution of mercury, onlypart of this metal affumes the metallic form,and the reft is oxyd combined with phofphorusand phofphoric acid.

From what has been obferv.ed above it isobvious that, the method of recovering one me-tal from its folution by the addition of another,which is fo often reforted to in analyfis, requiresconfulerable precaution, is liable to many inac-curacies, and can only be employed with pro-priety in certain cafes. The circumftanceswhich forbid its ufe, or render it inconvenient,,are the following. 1 ift._ Where the fait formedby the union of the precipitant in the acid isinfoluble in water, and will mix with the pre-cipitated metal from which it cannot be readily,feparated, as when fulphat of copper is decom-pofed by lead, antimony, or mercury, for thefulphats of thefe metals are little l'oluble in-water: or zndly, where the newly formed fait.

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