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acicl and a fmall portion of metal winch maybe feparated by evaporation or by an alkali.In like manner if fulphuric acid and mercuryare treated together, fulphureous acid is givenout, and the white mafs that remains is an uni-form acid fulphat, which deliquefces into adenfe folution. But if hot water be added,the turbeth or yellow fub-fulphat of mercury isprecipitated, and the folution contains an acidfulphat. Even the firft turbith may be furtherdecompofed by a frefh affufion of hot waterdividing it into a turbith Hill more loaded withoxyd, and an acid folution.

Thcfe, and many other fafts which mightbe adduced, illuftrate the general law of de-compofirion by precipitation, and the conftanttendency of metallic falts to divide into twocompounds, each confiding of both the condi-tuents of the fait but didributed in unequalproportions. Thefe phenomena are rendereddill more complex by the addition of new fub-dances, mod of which being defcribed underthe refpeHive metals, a repetition in this placewould be needlefs.

The fecond fpecies of metallic precipitatesare thofe in which the metal appears in its re-guline form. This takes place when one metalis precipitated by another, as when a piece ofiron is immerfed in a folution of copper, andalfo when phofphorus is kept for fome time incertain metallic dilutions.

Before the difcovery of oxygen, and of theoxydation of metals being neceffary to their fo-lution in acids, the precipitation of one metalby another was fuppofed to be produced byfirnple elective attraction, and the refpeftiveaffinities of the metals for the acids was putdown in the order of the precipitations whichthey produced. But this explanation thoughtrue in general, does not explain a number ofapparent anomalies in metallic precipitation, fothat we mud take into confideration the forceof other agents and other affinities.

The order of the precipitation of one metalby another is the following, among thofe thathave been the mod accurately examined, viz.zmc,' d iron, lead, tin, copper, filver, mercury,gold; that is to fay, zinc precipitates all theotner metals; iron precipitates all but zinc;lead all but zinc and iron, &c.

But fometimes the precipitation fails withdilutions iti one acid, though it fucceeds withOthers; thus zinc feparates iron in the metallicdate from muriat of iron, hut only as an oxyd

from the nitrat. A fmall excefs of acid is- ne-ceffary to begin the procefs in all cafes.

During this operation the following affinitiesmud atd, namely, that of the precipitant (ormetal by which the feparation is effefted) forthe oxygen of the metal already diffolved; thatof the oxyd thus produced, for the acid of thefolution; and to this Berthollet adds, that ofthe two metals for each other when in the re-guline date. The fum of thefe affinities mudovercome that which exids between the confti-tuent parts of the metallic fait intended to bedecompofed. b

It is poffible therefore that no precipitationmay take place even when the metal added hasa greater affinity for oxygen than that alreadydiffolved, if the oxyd of the former metal liasa lefs affinity for the add than the oxyd of thelatter has. And on the other hand a precipita-tion may be efieded even if the precipitant haslefs affinity for oxygen than the metal alreadydiffolved, provided its oxyd has a greater affi-nity for the acid. In this latter cafe howeverthere mud be a confiderable excefs of acid pre-fent in order to begin the oxydation of the pre-cipitant. Thus, again, it is explained why a pre-cipitation will take place with the folution inone acid, and not in another, fince the acidstliemfelves differ confiderably in their affinityfor the fame metallic oxyd, fo that in the firdcafe, this affinity may furpafs that of the fameacid for the metallic oxyd already diffolved, andin the fecond may fall fhort of it.

Still however (as Berthollet remarks) if theonly aftive affinities in thefe cafes were thofeof the metals for oxygen and the oxyds foracids, it is not probable that the reduction ofthe metal whofe affinities were the weaked,would be complete, but in all probability therewould be only an unequal partition of the acidand oxygen, fo as to produce the more oxygen-ated falts of one metal, and the lefs oxygenatedfalts of the other metal, which would beagreeable to the ufual mode of addion in fimilarcafes. Hence it becomes neceffary to- recur tothe well-known affinity which exids betweenthe metals themfelves, and many fa£ts {hewthat this is concerned in thefe procefi’es.

For example when a plate of polilhed copperis dipped into a folution of nitrated mercury, itis indantly whitened by the mercury precipi-tated upon its furlace, but this metal is notmerely depofited upon the copper, but amalgam-ated with it, fo that it cannot be feraped or

* Bergman. 11 Vauquelin, An. Cliim. xxviii. p. 4°-

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