(32.) a. Sulphydric acid, or sulphuretted hydrogen, reacts with the salts belonging to this group to form metallic sulphides, as shown by the following typical equations, in which M stands for an atom of metal:

In this manner the salts of perissad metals, as silver Ag', arsenic As", antimony Sb", and bismuth Bi'", are decomposed by sulphuretted hydrogen.

MCI, + H.S = 2HCl + M'S

In this manner the salts of artiad metals, as lead Pb", mercury Hg", copper Cu", cadmium Cd", tin (stannosum) Sn", and tin (stannicum) Sn"" are decomposed by sulphuretted hydrogen.

The sulphides thus produced differ much from one another as regards their solubility in mineral acids. They are all completely dissolved by nitro-muriatic acid; except that of silver, which is converted into insoluble chloride of silver; and that of lead, which is converted partly into the sparingly soluble chloride, partly into the insoluble sulphate of lead, owing to an oxidation of its constituent sulphur.

Hot nitric acid dissolves the sulphides of arsenic, bismuth, silver, copper, and cadmium, but has no appreciable action on the sulphides of mercury. It converts the sulphides of tin and antimony into their insoluble oxides or anhydrides, SnO, and Sb204 respectively. When slightly diluted, it dissolves sulphide of lead completely, but otherwise it converts a portion of it into insoluble sulphate of lead. The action of nitric acid upon the sulphides is generally attended with a separation of sulphur, which, on boiling, gradually assumes the form of melted globules.

Strong hydrochloric acid at a boiling temperature has no action on the sulphides of arsenic and mercury. It converts the sulphides of silver and lead into their insoluble or sparingly

soluble chlorides, and dissolves the remaining sulphides of the group with greater or less facility.

(33.) 6. The disulphide of tin, and the trisulphides of arsenic and antimony, unite with the sulphides of alkali-metal to form soluble sulphur-salts corresponding to the well-known oxygen salts, thus:

Hence these sulphides are distinguished from the remainder by their solubility in sulphide of ammonium.* Protosulphide of tin is not soluble in pure colourless sulphide or sulphydrate of ammonium; but it is soluble in the ordinary yellow solution of the persulphide, whereby it becomes converted into the abovedescribed compound of disulphide of tin, thus:

(NH4)2S, + SnS

=

(NH4)2S.SnS, or (NH4)2SnS3.

On the addition of hydrochloric acid to any of these sulphosalts, they are decomposed with reprecipitation of their respective sulphides, thus:

(NH4)2S.SnS, + 2HCl = 3(NH4)2S.As2S, + 6HCI =

H2S + SnS2 + 2NH CI 3H2S + As2S3+ 6NH4Cl.

It is advisable not to dissolve the sulphides of this sub-section in an unnecessarily large quantity of yellow sulphide of ammonium, lest the subsequent addition of hydrochloric acid should separate so great a quantity of whitish sulphur as to conceal the colour of the reprecipitated sulphides.

7. Sulphide of arsenic is distinguishable from the sulphides of tin and antimony by its insolubility even in boiling hydrochloric acid; and by its solubility in a warm solution of sesquicarbonate of ammonia.

* Sulphide of copper is quite insoluble in the sulphides of sodium and potassium; but is slightly soluble in sulphide of ammonium, especially when it contains, as it usually does, some free ammonia.

Sulphide of antimony and persulphide of tin are distinguishable from one another by their difference in colour. When pure they dissolve completely in hydrochloric acid; but as usually obtained they often contain excess of sulphur, which remains undissolved. Upon evaporating down their hydrochloric acid solutions to a small bulk, stannic and antimonious chlorides are obtained respectively. The former chloride does not have its transparency affected by dilution, neither does the diluted liquid yield any deposit upon a surface of metallic tin: and again, an acid solution of chloride of tin, in which a small fragment of zinc has been dissolved, gives with corrosive sublimate a white precipitate of calomel, gradually becoming grey from its conversion into metallic mercury (vide par. 34).

The latter chloride is generally rendered opaque by diluting its solution, which again becomes clear on the addition of tartaric acid; while the diluted liquid yields an abundant black deposit of pulverulent antimony upon a surface of metallic tin. Moreover, chloride of antimony reacts satisfactorily when examined by Marsh's or Reinsch's process.

Solid compounds of arsenic are most readily recognised by the reduction test (vide par. 66).

(34.) 8. Potash reacts with the salts belonging to the second section of this group, to precipitate the hydrated oxides of the respective metals, thus:

2BiCl, + 6KHO = 6KCI +

The hydrated oxides of mercury, lead, copper, and cadmium are produced according to the following general equation :

MCI, + 2KHO = 2KC1 + MO.H2O.

Hydrate of lead is soluble in excess of potash; the hydrates of silver, copper and cadmium are soluble in excess of ammonia; while those of bismuth and mercury are insoluble in either reagent. Independently of their behaviour with sulphuretted hydrogen and

caustic alkalis, the metals of this sub-section of the first group are characterised by the following reactions.

Bismuth solutions, unless too acid, when they must first be evaporated down, yield an opaque white precipitate on the addition of water, due to the formation of some insoluble basic salt, thus:

These basic salts of bismuth are insoluble in tartaric acid, and are blackened by sulphuretted hydrogen or sulphide of ammonium.

Mercury solutions yield with protochloride of tin a white precipitate becoming grey spontaneously, or more rapidly on the application of heat. The white precipitate is calomel, which is formed from mercurous salts by double decomposition, thus:

2HgNO3 + + SÁCH,

==

2 HgCl + Sn(NO3), or SnO.N2O,.

But it is formed from mercuric salts by reduction, thus:

2HgCl, + SÁCH,

=

2HgCl + Sách

The grey deposit consists of finely divided metallic mercury, produced by an abstraction of chlorine from the calomel first precipitated:

2 HgCl + SẮC,

=

Hga + SnCl4.

This grey deposit, when boiled with hydrochloric acid, acquires the characteristic appearance of globules of mercury.

Lead solutions yield with sulphuric acid, or soluble sulphates, a white precipitate of sulphate of lead, insoluble in cold nitric or hydrochloric acid:

Pb(NO3)2 + H2SO4

=

2HNO3 + PbSo or PbO.SO.

The precipitate is distinguished from the similar precipitate

produced with barium- and strontium-salts, by its solubility in excess of potash, by its solubility in boiling hydrochloric acid, and by its becoming blackened by sulphuretted hydrogen or sulphide of ammonium.

Silver solutions yield with hydrochloric acid or soluble chlorides a white clotty precipitate of chloride of silver:

The precipitate is soluble in ammonia, but insoluble in the strongest nitric acid, even when boiling. It is turned of a slatepurple colour by exposure to light.

Copper solutions, even when very dilute, give with ferrocyanide of potassium a chocolate-red precipitate of ferrocyanide of copper, or of ferrocyanide of copper and potassium, thus:

2CuSO4 + K4FeCу6

=

2K2SO4 + Cu.FeCy6.

The precipitate is turned of a pale blue colour by potash, and is then readily soluble in ammonia, forming a deep purple coloured liquid, by which properties it is distinguished from the similarly coloured ferrocyanide of uranium.

Cadmium solutions are specially recognised by the bright yellow colour and insolubility in sulphide of ammonium, of the precipitated sulphide of cadmium Cds, produced by sulphuretted hydrogen or sulphide of ammonium. Of all the sulphides of the first group of metals, sulphide of cadmium is the one most readily soluble in acids. Cadmium-salts, moreover, are readily identified by their behaviour before the blowpipe.

§ IV.-EXAMINATION FOR BASES OF GROUP II.

(35.) The members of this group are precipitated by sulphide or sulphydrate of ammonium, but are not precipitated from their acidified solutions by sulphuretted hydrogen. Inasmuch as sulphide of ammonium also precipitates most of the metals of the first group, their absence must be ascertained by the non-produc