[42 bis] Lead sulphide in the presence of zinc and hydrochloric acid is completely reduced to metallic lead, all the sulphur being given off as hydrogen sulphide.
[43] Lead sulphate, PbSO4, occurs in nature (anglesite) in transparent brilliant crystals which are isomorphous with barium sulphate, and have a specific gravity of 6·3. The same salt is formed on mixing sulphuric acid or its soluble salts with solutions of lead salts, as a heavy white precipitate, which is insoluble in water and acids, but dissolves in a solution of ammonium tartrate in the presence of an excess of ammonia. This test serves to distinguish this salt from the similar salts of strontium and barium.
[44] According to J. B. Hannay (1894) the last named decomposition (PbS + PbSO4 = 2Pb + 2SO2) is really much more complicated, and in fact a portion of the PbS is dissolved in the Pb, forming a slag containing PbO, PbS and PbSO4, whilst a portion of the lead volatilises with the SO2 in the form of a compound PbS2O2, which is also formed in other cases, but has not yet been thoroughly studied.
Besides these methods for extracting lead from PBS in its ores, roasting (the removal of the S in the form of SO2) and smelting with charcoal with a blast in the same manner as in the manufacture of pig iron (Chapter [XXII.]) are also employed.
We may add that PbS in contact with Zn and hydrochloric acid (which has no action upon PbS alone) entirely decomposes, forming H2S and metallic lead: PbS + Zn + 2HCl = Pb + ZnCl2 + H2S.
As lead is easily reduced from its ores, and the ore itself has a metallic appearance, it is not surprising that it was known to the ancients, and that its properties were familiar to the alchemists, who called it ‘Saturn.’ Hence metallic lead, reduced from its salts in solution by zinc, having the appearance of a tree-like mass of crystals, is called ‘arbor saturni,’ &c.
[45] Freshly laid new lead pipes contaminate the water with a certain amount of lead salts, arising from the presence of oxygen, carbonic acid, &c., in the water. But the lead pipes under the action of running water soon become coated with a film of salts—lead sulphate, carbonate, chloride, &c.—which are insoluble in water, and the water pipes then become harmless.
[46] Lead is used in the arts, and owing to its considerable density, it is cast, mixed with small quantities of other metals, into shot. A considerable amount is employed (together with mercury) in extracting gold and silver from poor ores, and in the manufacture of chemical reagents, and especially of lead chromate. Lead chromate, PbCrO4, is distinguished for its brilliant yellow colour, owing to which it is employed in considerable quantities as a dye, mainly for dyeing cotton tissues yellow. It is formed on the tissue itself, by causing a soluble salt of lead to react on potassium chromate. Lead chromate is met with in nature as ‘red lead ore.’ It is insoluble in water and acetic acid, hut it dissolves in aqueous potash. The so-called pewter vessels often consist of an alloy of 5 parts of tin and 1 part of lead, and solder is composed of 1 to 2 parts of tin with ½ part of lead. Amongst the alloys of lead and tin, Rudberg states that the alloy PbSn3 stands out from the rest, since, according to his observations, the temperature of solidification of the alloy is 187°.
[47] The normal lead acetate, known in trade as sugar of lead, owing to its having a sweetish taste, has the formula Pb(C2H3O2)2,3H2O. This salt only crystallises from acid solutions. It is capable of dissolving a further quantity of lead oxide or of metallic lead in the presence of air. A basic salt of the composition Pb(C2H3O2)2,PbH2O2 is then formed which is soluble in water and alcohol. As in this salt the number of atoms is even and the same as in the hydrate of acetic acid, C2H4O2,H2O = C2H3(OH)3, it may be represented as this hydrate in which two of hydrogen are replaced by lead—that is, as C2H3(OH)(O2Pb). This basic salt is used in medicine as a remedy for inflammation, for bandaging wounds, &c., and also in the manufacture of white lead. Other basic acetates of lead, containing a still greater amount of lead oxide, are known. According to the above representation of the composition of the preceding lead acetate, a basic salt of the composition (C2H3)2(O2Pb)3 would be also possible, but what appear to be still more basic salts are known. As the character of a salt also depends on the property of the base from which it is formed, it would seem that lead forms a hydroxide of the composition HOPbOH, containing two water residues, one or both of which may be replaced by the acid residues. If both water residues are replaced, a normal salt, XPbX, is obtained, whilst if only one is replaced a basic salt, XPbOH, is formed. But lead does not only give this normal hydroxide, but also polyhydroxides, Pb(OH),nPbO, and if we may imagine that in these polyhydroxides there is a substitution of both the water residues by acid residues, then the power of lead for forming basic salts is explained by the properties of the base which enters into their composition.
[48] Few compounds are known of the lower type PbX, and still fewer of the intermediate type PbX3. To the first type belongs the so-called lead suboxide, Pb2O, obtained by the ignition of lead oxalate, C2PbO4, without access of air. It is a black powder, which easily breaks up under the action of acids, and even by the simple action of heat, into metallic lead and lead oxide. This is the character of all suboxides. They cannot be regarded as independent salt-forming oxides, neither can those forms of oxidation of lead which contain more oxygen than the oxide of lead, PbO, and less than the dioxide, PbO2. As we shall see, at least two such compounds are formed. Thus, for example, an oxide having the composition Pb2O3 is known, but it is decomposed by the action of acids into lead oxide, which passes into solution, and lead dioxide, which remains behind. Such is red lead. (See further on.)