strong hydrochloric acid upon PbO_{2}, or, in the cold, by passing a stream of chlorine through water containing PbCl_{2} in suspension. The resultant yellow solution gives off chlorine when heated. With a solution of sal ammoniac (Nicolukin, 1885) it gives a precipitate of a double salt, (NH_{4})_{2}PbCl_{6} (very slightly soluble in a solution of sal ammoniac), which when treated with strong sulphuric acid (Friedrich, 1890) gives PbCl_{4} as a yellow liquid sp. gr. 3·18, which solidifies at -18°, and when heated gives PbCl_{2} + Cl_{2}. It is not acted upon by H_{2}SO_{4} like SnCl_{4}. Tetrafluoride of lead (Brauner) belongs to the same class of compounds, it easily forms double salts and decomposes with the evolution of fluorine (Chapter II., Note 49 bis).[56 bis] [55] Lead dioxide is often called lead peroxide, but this name leads to error, because PbO_{2} does not show the properties of true peroxides, like hydrogen or barium peroxides, but is endowed with acid properties--that is, it is able to form true salts with bases, which is not the case with true peroxides. Lead dioxide is a normal salt-forming compound of lead, as Bi_{2}O_{5} is for bismuth, CeO_{2} for cerium, and TeO_{3} for tellurium, &c. They all evolve chlorine when treated with hydrochloric acid, whilst true peroxides form hydrogen peroxide. The true lead peroxide, if it were obtained, would probably have the composition Pb_{2}O_{5}, or, in combination with peroxide of hydrogen, H_{2}Pb_{2}O_{7} = H_{2}O_{2} + Pb_{2}O_{5}, judging from the peroxides corresponding with sulphuric, chromic, and other acids, which we shall afterwards consider. As a proof of the fact, that the form PbO_{2}, or PbX_{4}, is the highest normal form of any combination of lead, it is most important to remark that it might be expected that the action of lead chloride, PbCl_{2}, on zinc-ethyl, ZnEt_{2}, would result in the formation of zinc chloride, ZnCl_{2}, and lead-ethyl, PbEt_{2}, but that in reality the reaction proceeds otherwise. Half of the lead is set free, and lead tetrethyl, PbEt_{4}, is formed as a colourless liquid, boiling at about 200° (Butleroff, Frankland, Buckton, Cahours, and others). The type PbX_{4} is not only expressed in PbEt_{4} and PbO_{2}, but also in PbF_{4}, obtained by Brauner. [56] According to Carnelley and Walker, the hydrate (PbO_{2})_{3},H_{2}O is then formed; it loses water at 230°. The anhydrous dioxide remains unchanged up to 280°, and is then converted into the sesquioxide, Pb_{2}O_{3}, which again loses oxygen at about 400°, and forms red lead, Pb_{3}O_{4}. Red lead also loses oxygen at about 550°, forming lead oxide, PbO, which fuses without change at about 600°, and remains constant as far as the limit of the observations made (about 800°).