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however, much more rapidly when the filver isalloyed (as in the common plate) than whenpure. In general the tarniJJj of filver is in toofinal 1 a quantity to be examined, but when thismetal has been expofed for a very long feries ofyears to the common air of towns and inhabitedplaces, the corrofion penetrates deeper,'and thealtered part will readily peel off. This how-ever is not the pure oxyd, but the fulphuret, aswas afeertained by Proult, the properties ofwhich will be afterwards deferibed, fo that weknow of no Ample oxyd of filver formed fpon-taneoufly in the air.

Water has no effect whatever on filver at anytemperature, and is not decompofed upon it.But when bright clean filver is immerfed incommon natural waters, particularly thofe thatreceive the refufe of animal or vegetable matter,in no great length of time the whole becomestarn idled and brittle, and is converted into afulphuret.™

Though filver at no temperature undergoescombuftion by fimple contact of common air,or oxygen gas, yet it burns totally and moltbeautifully in the eleftric and efpecially thegalvanic circles. Silver wire or leaf appliedto a powerful galvanic apparatus, burns mollreadily with an exquifite green flame, and adenfe grey fmoke.

The fulphuric acid has no effect on filver inthe cold in any date of dilution, but when theacid is concentrated and a boiling heat is em-ployed, effervefcence begins, an abundance ofvery pure fulphureous acid gas is given out (aswith this acid and mercury) and the metal ischanged to a white pulverulent mafs, or is dif-folved entirely into a denfe clear liquid if thequantity of the acid is about four times that ofthe filver, which latter Ihould be added infmall Ihreds or grains.

This folution is extremely ftyptic and re-quires an excefs of acid. By evaporation itfurnilhes fmall white brilliant needled cryftals."

Sulphat of filver when heated per fe, meltsand fwells up, and is not changed in a mode-rate heat, but, on increafing it, blackens, ex-hales fulphureous acid and oxygen, and finallyreturns to the pure metallic Hate. This faitis flowly difcoloured by light, and turns of ablackifh brown. None of the acids except themuriatic alter it, but this, in whatever com-bination it be, feizes upon the whole of thefilver and unites with it into that curdy fub-ftance, the muriat of filver or luna cornea,which will be prefently deferibed.

• See Wifeman and Hatchett in Phil. Trans.' vol.

With alkalies and other decompefing fub-ftances the fulphat of filver is decompofednearly as the nitrat. Sulphat of filver is alfoformed by adding to nitrat of filver either ful-phuric acid or any alkaline or earthy fulphat.The relult in either of thefe cafes is a whiteheavy precipitate of fulphat of filver which isinfolubie in water, but with a frefh portion offulphuric acid forms a folution fimilar to thatalready deferibed.

The precipitated oxyd of filver diflolves moreeafily in fulphuric acid than the metal. s

The fulphureous acid has no action on filverbut readily combines with its oxyd. This fo-lution cryllallizes fpontaneoufly in fmall pearl-grey brilliant grains, unaltered by light. 0 Thefame fait is procured (as in the former cafe) byadding to the nitrat of filver either fulphureousacid or a fulphite of potafh or foda. The ful-phite of ammonia diflolves the oxyd of filver,which it detaches from its combination withthe nitric acid and (agreeably to the ufual habi-tudes of the ammoniacal falts) produces a tripleammoniaco-fulphite of filver, which by expofureto the air becomes converted into a fulphat.

The nitric acid diflolves filver eafily, and inlarge quantity, and is the acid conftantly em-ployed for this purpofe in the arts. The con-centrated acid fhould be diluted with from twoto four parts of water. Very foon after thefilver is immerfed in the acid a ftrong effervef-cence begins, much nitrous gas is given out,which forms a copious orange-coloured vapouras it efcapes from the veffel, and the acid af-fumes a light blue-green colour which is inde-pendant of any copper which the filver maycontain, and is owing to the folution of a por-tion of the nitrous gas in the acid. If howeverthe filver contains copper this increafes thebluenefs, and the colour remains after the fo-lution is cold and faturated, which is not thecafe when the filver is pure. A confiderableheat is excited by the action of the acid on thefilver, which alfo much a {lilts in the rapidityof the procefs, fo that where the quantity ofeach is at all confiderable no artificial heat isrequired. The folution is made very conveni-ently by putting the filver (granulated or inIhreds) into a decanter or matrafs, adding theacid, and to it about four parts or more of hotwater, and fetting it under a chimney to drawoff the fumes. The folution then begins almoflimmediately, and goes on with increafing vio-lence for a time and then continues fteadily tillthe filver is diffolved or the acid faturated,

Ixxxviii. • Beaume. • Fourcroy.