[50] It is very instructive to observe that lead not only easily forms basic salts, but also salts containing several acid groups. Thus, for example, lead carbonate occurs in nature and forms compounds with lead chloride and sulphate. The first compound, known as _corneous lead_, _phosgenite_, has the composition PbCO_{3},PbCl_{2}; it occurs in nature in bright cubical crystals, and is prepared artificially by simply boiling lead chloride with lead carbonate. A similar compound of normal salts, PbSO_{4},PbCO_{3}, occurs in nature as _lanarkite_ in monoclinic crystals. _Leadhillite_ contains PbSO_{4},3PbCO_{3}, and also occurs in yellowish, monoclinic, tabular crystals. We will turn our attention to these salts of lead, because it is very probable that their formation is allied to the formation of the basic salts, and the following considerations may lead to the explanation of the existence of both. In describing silica we carefully developed the conception of polymerisation, which it is _also indispensable to recognise in the composition of many other oxides_. Thus it may be supposed that PbO_{2} is a similar polymerised compound to SiO_{2}--_i.e._ that the composition of lead peroxide will be Pb_{_n_}O_{2_n_}, because lead methyl, PbMe_{4}, and lead ethyl, PbEt_{4}, are volatile compounds, whilst PbO_{2} is non-volatile, and is very like silica in this respect, and not in the least like carbonic anhydride. Still more should a polymeric structure, Pb_{_n_}O_{_n_}, be ascribed to lead oxide, since it differs as little from lead dioxide in its physical properties as carbonic oxide does from carbonic anhydride, and being an unsaturated compound is more likely to be capable of intercombination (polymerisation) than lead dioxide. These considerations respecting the complexity of lead oxide could have no real significance, and could not be accepted, were it not for the existence of the above-mentioned basic and mixed salts. The oxide apparently corresponds with the composition Pb_{_n_}X_{2_n_}, and since, according to this representation, the number of X's in the salts of lead is considerable, it is obvious that they may be diverse. When a part of these X's is replaced by the water residue (OH) or by oxygen, X_{2} = O, and the other parts by an _acid residue_, X, then basic salts are obtained, but if a part of the X's is replaced by acid residues of one kind, and the other part by acid residues of another kind, then those mixed salts about which we are now speaking are formed. Thus, for example, we may suppose, for a comparison of the composition of the majority of the salts of lead, that _n_ = 12, and then the