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( 310 ’ )

changes the colour of litmus blue, and givesa blue lake with it, but the nitrat produces noefteCh the nitrat turns cochineal fcarlet; thefub-nitrat, deep yellow: the nitrat does notchange fulphat of indigo; but the fub-nitratdifcolours it entirely, and the filver is reduced.Ammonia gives a black precipitate with thefub-nitrat which is limply reguline filver inminute divifion and does not fulminate; nor isthe precipitate redifiolved by an excefs of am-monia, but the clear liquor contains oxyd offilver diflolved in this alkali. When the folu-tion of the fub-nitrat is expofed to the air, in afew days it lofes its yellow colour, and depofitslarge blades of cryftallized nitrat of filver.The cryftais are then in the ftate of commonnitrat, fo that the two falts differ both in thequantity of acid and of oxygen being lefs inthe fub-nitrat. If fome drops of nitric acidare poured into the concentrated folution ofthe fub-nitrat large cryftais of nitrated filverform almoft immediately, the yellow colour isloft, and the whole becomes common nitrat.If the folution is lefs concentrated, and nitricacid is added, nitrous gas is given out.

If muriatic acid is added to the fub-nitrat,common muriat of filver is precipitated, fo thatthe fupernatant liquor ought to be ftill lefsoxygenated than before.

Water afts in a ftriking manner on the folidfub-nitrat. If cold water is poured on it apart only dilfolves, and the refidue is a yellowpowder, a compound of filver, nitric acid, andoxygen, in which the two latter fubftances arein hill fmaller proportion to the metal than inthe liquid fub-nitrat, or the folid fait beforethe afl'ufion of water. The fait therefore di-vides itfelf into two portions, the foluble partabftraCting from the other a portion of its acidand its oxygen, which is nearly fimilar to whattakes place with the fulphats of mercury dur-ing the formation of turbith, and with the nitratof this metal. If boiling water is added to thefolid fub-nitrat, thefe changes proceed ftill fur-ther and the infoluble portion becomes fuccef-fively, but almoft inftantaneoufly, yellow, red,and black. If whilft it is yellow or red, a fewdrops of nitric acid are poured in, the furtherchange is flopped and the whole diffolves intoa colourlefs folution, but after the colour hasbecome black, this does not take place.. Thisblack powder is merely filver in fine divifion,and filvers the bottom of the veffel by Handing.

If the folid fub-nitrat is ftill furth:r heatedin a retort, it thickens, gives out nitrous gas,

fufes, and a yellow fublimate arifes. No fuchfublimate is formed with the nitrat. The abovefa&s (hew that there are at leaft two ftates ofoxydation of filver wdien in folution, but theingenious difeoverer does not give the propor-tions of the fub-oxyd.

All the folutions of filver are blackened im-mediately by fulphuretted hydrogen, either inthe gaffeous or liquid ftates, and all the folidfulphurets produce the fame change on filver,as will be prefently mentioned.

A compound acid, extremely ufeful for dif-folving filver when mixed with copper andfome other metals, has been difeovered by Mr.Keir. u It is formed by diffolving one part ofnitre in about eight or ten parts by weight offtrong fulphuric acid. When pieces of filverare thrown into this acid (undiluted withwater) and a heat of from ioo° to 2 oo° isapplied, an effevvefcence of nitrous gas takesplace and the filver is diflolved. This makes avery denfe heavy folution which readily con-cretes by cooling, but may be moderately di-luted with water without becoming turbid.This compound acid (which may be termedthe nitro-fulphuric) diffolves about a fifth or afixth of its weight of filver, and with muchmore eafe than the fulphuric acid fingly woulddo. The quantity of nitrous gas given out inthe procefs feems to be lefs in proportion asthat of the nitre is greater. The nitro-fulphuricacid, undiluted, alfo oxydates and partly dif-folves tin and mercury, but it hardly touchescopper, lead, gold, or iron. Hence it is par-ticularly ufeful in recovering the filver fromthe furface of any filver plated metal, which isan objeCf in large manufactures of platedgoods, and was ufually done from copper plateat Birmingham by more expenfive methods.The precife proportions of fulphuric acid andnitre are not of much confequence. On theother hand if water is added to the acid, itspower of diffolving filver is much leffened,and it then afts on copper with eafe.

The filver may be recovered from the folu-tion either by adding common fait which fepa-rates it in the form of muriat of filver ("whichis afterwards reftored to the metallic ftate byfufion with alkalies) or, more Amply, by dilut-ing the folution with water and continuing toimmerfe the fame pieces of copper plate whichhave loft their filvering. The acid being nowable to aft on the copper will faturate itfelfwith this metal in preference, and will depofitthe filver in the metallic form. The fulphat

« Phil. Tranf. vol. So,