[54 bis] Frémy obtained potassium plumbate in the following manner. Pure lead dioxide is placed in a silver crucible, and a strong solution of pure caustic potash is poured over it. The mixture is heated and small quantities are removed from time to time for testing, which consists in dissolving in a small quantity of water and decomposing the resultant solution with nitric acid. There is a certain moment during the heating when a considerable amount of insoluble lead dioxide is precipitated on the addition of the nitric acid; the solution then contains the salt in question, and the heating must be stopped, and a small amount of water added to dissolve the potassium plumbate formed. On cooling the salt separates in somewhat large crystals, which have the same composition as the stannate--that is, PbO(KO)_{2},3H_{2}O. Minium is the first and most ordinary means of producing _lead dioxide_, or plumbic anhydride, PbO_{2},[55] because when red lead is treated with dilute nitric acid it gives up lead oxide, and PbO_{2} remains, on which dilute nitric acid does not act. The composition of minium is Pb_{3}O_{4}, and therefore the action of nitric acid on it is expressed by the equation: Pb_{3}O_{4} + 4HNO_{3} = PbO_{2} + 2Pb(NO_{3})_{2} + 2H_{2}O. The dioxide may also be obtained by treating lead hydroxide suspended in water with a stream of chlorine. Under these conditions the chlorine takes up the hydrogen from the water, and the oxygen passes over to the lead oxide.[56] When a strong solution of lead nitrate is decomposed by the electric current, the appearance of crystalline lead dioxide is also observed upon the positive pole; it is also found in nature in the form of a black crystalline substance having a specific gravity of 9·4. When artificially produced it is a fine dark powder, resisting the action of acids, but nevertheless when treated with strong sulphuric acid it evolves oxygen and forms lead sulphate, and with hydrochloric acid it evolves chlorine. The oxidising property of lead dioxide depends of course on the facility of its transition into the more stable lead oxide, which is easily understood from the whole history of lead compounds. In the presence of alkalis it transforms chromium oxide into chromic acid, whilst lead chromate, PbCrO_{4}, is formed, remaining, however, in solution, on account of its being soluble in caustic alkalis. The oxidising action of lead dioxide on sulphurous anhydride is most striking, as it immediately absorbs it, with formation of lead sulphate. This is accompanied by a change of colour and development of heat, PbO_{2} + SO_{2} = PbSO_{4}. When triturated with sulphur the mixture explodes, the sulphur burning at the expense of the oxygen of the lead dioxide. _Tetrachloride of lead_, PbCl_{4}, belongs to the same class of lead compounds as PbO_{2}. This chloride is formed by the action of