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diffolved in fifty times its weight of water, andan excefs of potafh or foda is added, no preci-pitation takes place, fo that the acid continuesin part to a£t upon the lime and renders itfoluble in a much lefs quantity of water thanthe lime alone would require for folution.

It may be afl'umed therefore as a generalprinciple, and one which is of great importancein the explanation of chemical decompofition,that where a fait is compofed of an acid and abafe, infoluble or little foluble by itfelf, thedecompofition effedted by any foluble bafe isthe refult not of a ‘ fimple but a compoundadlion, in which on the one hand the decom-poling bafe-unites with more or lefs of the acidand remains in folution, whilft the other baferefumes its infolubility by the feparation of partof its acid, and forms a precipitate with theremainder. This will be found to apply itillmore ftrikingly in the formation of metallicprecipitates, and of thofe falts which (like thefub-fulphat of mercury or turbith) -were former-ly taken for fimple oxyds, but are now foundto be falts with excefs of oxyd.

Chemical affinity is in many cafes determinedby precipitation, but a contradiction in themode of applying this principle has been wellpointed out by Berthollet .

The orders of affinity which the feveral bafeshave for the acids is ufually determined byadding any bafe to a known combination of anacid and another bafe, and if the latter bafeis precipitated it has been inferred that itsaffinity for the acid is lefs than that of the otherbafe. Thus when lime is precipitated by addingpotafh to muriat of lime, it is inferred thatpotafh has a -ftronger affinity for muriatic acidthan lime, and hence the preference in theorder of affinity has been given to the moftfoluble bafes. But on the other hand, therefpeClive affinities of the acids has been deter-mined in the contrary way, as for example,when oxalic acid is added to muriat of lime aprecipitate of oxalat of lime is produced, whencethe fuperior affinity of the oxalic acid over themuriatic for lime has been deduced. Thus inthefe cafes the preference in affinity has beengiven to thofe acids which have the molt dif-pofition to form infoluble combinations.

But many inftances may be given to ffiewthat the phenomena of precipitation are diftinftfrom thofe of chemical affinity, fo that therefulting infolubility or force of cohefion whichproduces precipitations is fometimes fuper-added to the fimple force of affinity, and at

other times oppofed to it, and ought in ftri&nelsto be always confidered as a diftindt principle.

Of Metallic Precipitates.

The metal of every metallic folution in anyacid is always in the ftate of an oxyd, fo thatthe folution mull contain at leaf!; three fubflan-ces, the acid, the metal, and oxygen unitedwith the metal. But the proportion of thelatter is fubjedt to a great number of variations,and is different both with the feveral metals andwith the fame metal in different circumftances.

A metallic folution may be decompofed andits metallic part precipitated either in thereguline ftate or as an oxyd, and thefe twomethods, which have each been adopted in theanalyfis of metallic bodies, require to be con-fidered feparately.

When a carbonated alkali is added to ametallic folution, the precipitate is a carbonatedoxyd of the metal, and like the other carbonatseffervefces on the addition of a ftronger acid.But when a pure alkali is added only to fatura-tion, the precipitate is in fome inftances nearlya pure oxyd, but in the greater number is anoxyd retaining a fmall portion of the acid withwhich it was united. This acid however is notfufficient to render it foluble and it may begenerally entirely taken away by fubfequentdigeftion in a concentrated fixed alkali providedthe oxyd is not foluble therein.

The firft action of alkalies therefore on moftmetallic folutions is to caufe an unequal divifionof the acid, the greater part of which uniteswith the alkali, and the remainder falls downalong with the oxyd now rendered infoluble.If ammonia be the alkali employed, there isalfo in general a partition of the alkali, and theprecipitate is a quadruple compound, of themetal, the oxygen united with it, and a fmallportion of the acid, and of the ammonia j andthe folution contains the greater part of theacid, of the alkali, and often a fmall part ofthe metallic oxyd fufficient to be very fenfibleto the tafte, and to chemical tells. Accordingto Prouft, lime alfo when ufed as a precipitantenters in a fmall proportion into the precipitate.

Metallic folutions fometimes yield precipi-tates on the mere addition of water, by thefimple diflribution of the conflituents into twoportions, a foluble and an infoluble one, theformer of which has an excefs of acid, and thelatter of oxyd. Thus when the clear nitrat ofbifmuth is diluted with water a white precipitatefalls down which is chiefly oxyd of bifmuth,and the fupernatant liquor contains an excefs of