[43] Lead sulphate, PbSO_{4}, 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 + PbSO_{4} = 2Pb + 2SO_{2}) 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 PbSO_{4}, whilst a portion of the lead volatilises with the SO_{2} in the form of a compound PbS_{2}O_{2}, 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 SO_{2}) 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 H_{2}S and metallic lead: PbS + Zn + 2HCl = Pb + ZnCl_{2} + H_{2}S. 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.