[51] A similar basic salt having a white colour, and therefore used as a substitute for white lead, is also obtained by mixing a solution of basic lead acetate with a solution of lead chloride. Its formation is expressed by the equation: 2PbX(OH),PbO + PbCl_{2} = 2Pb(OH)Cl,PbO + PbX_{2}. Similar basic compounds of lead are met with in nature--for instance, _mendipite_, PbCl,2PbO, which appears in brilliant yellowish-white masses. The ignition of red lead with sal-ammoniac results in similar polybasic compounds of lead chloride, forming the _Cassel's_, or _mineral yellow_ of the composition PbCl_{2}_n_PbO. _Lead iodide_, PbI_{2}, is still less soluble than the chloride, and is therefore obtained by mixing potassium iodide with a solution of a lead salt. It separates as a yellow powder, which may be dissolved in boiling water, and on cooling separates in very brilliant crystalline scales of a golden yellow colour. The salts PbBr_{2}, PbF_{2}, Pb(CN)_{2}, Pb_{2}Fe(CN)_{6} are also insoluble in water, and form white precipitates. Lead carbonate, or _white lead_, is the most extensively used basic lead salt. It has the valuable property of 'covering,' which only to a certain extent appertains to lead sulphate and other white powdery substances used as pigments. This faculty of 'covering' consists in the fact that a small quantity of white lead mixed with oil spreads uniformly, and if such a mixture be spread over a surface (for instance, of wood or metal) the surface is quickly covered--that is, light does not penetrate through even a very thin layer of superposed white lead; thus, for example, the grain of the wood remains invisible.[52] White lead, or _basic lead carbonate_, after being dried at 120°, has a composition Pb(OH)_{2},2PbCO_{3}.[53] It may be obtained by adding a solution of sodium carbonate to a solution of one of the basic salts of lead--for instance, the basic acetate--and likewise by treating this latter with carbonic acid. For this purpose the solution of basic acetate is poured into the vessel _f_; it is prepared in the vat A, containing litharge, into which the pump P delivers the solution of the acetate, which remains after the action of carbonic anhydride on the basic salt. In A a basic salt is formed having a composition approaching to Pb_{4}(OH)_{6}(C_{2}H_{3}O_{2})_{2}; carbonic anhydride, 2CO_{2}, is passed through this solution and precipitates white lead, Pb_{2}(OH)_{2}(CO_{3})_{2}, and normal lead acetate, Pb(C_{2}H_{3}O_{2})_{2}, remains in the solution, and is pumped back into the vat A containing lead oxide, where the normal salt is again (on being agitated) converted into the basic salt. This is run into the vessel E, and thence into _f_. Into the latter carbonic anhydride is delivered from the generator D, and forms a precipitate of white lead.[53 bis]