This substance was obtained by Spring (1889) in the form of a hydrate, H_{2}Sn_{2}O_{7} = 2(SnO_{3})H_{2}O, by mixing a solution of SnCl_{2}, containing an excess of HCl, with freshly prepared peroxide of barium. A cloudy liquid is then obtained, and this after being subjected to dialysis leaves a gelatinous mass which on drying is found to have the composition Sn_{2}H_{2}O_{7}. Above 100° this substance gives off oxygen and leaves SnO_{2}. It is evident that SnO_{3} bears the same relation to SnO_{2} as H_{2}O_{2} to H_{2}O or ZnO_{2} to ZnO, &c. Tin occupies the same position amongst the analogues of silicon as cadmium and indium amongst the analogues of magnesium and aluminium respectively, and as in each of these cases the heavier analogues with a high atomic weight and a special combination of properties--namely, mercury and thallium--are known, so also for silicon we have _lead_ as the heaviest analogue (Pb = 206), with a series of both kindred and special properties. The higher type, PbX_{4}--for instance, PbO_{2}--is in a chemical sense far less stable than the lower type, PbX. The ordinary compounds of lead correspond with the latter, and in addition to this, PbO, although not particularly energetic, is still a decided base easily forming basic salts, PbX_{2}(PbO)_{n}. Although the compounds PbX_{4}, are unstable they offer many points of analogy with the corresponding compounds of tin SnO_{2}; this is seen, for instance, in the fact that PbO_{2} is a feeble acid, giving the salt PbK_{2}O_{3}, that PbCl_{4} is a liquid like SnCl_{4} which is not affected by sulphuric acid, and that PbF_{4} gives double salts, like SnF_{4} or SiF_{4} (Brauner 1894. See Chapter II., Note 49 bis); Pb(C_{2}H_{5})_{4} also resembles Sn(C_{2}H_{5})_{4} &c. All this shows that lead is a true analogue of tin, as Hg is of cadmium.[41 bis] [41 bis] Although this has long been generally recognised from the resemblance between the two metals, still from a chemical point of view it has only been demonstrated by means of the periodic law.