[29] As the colloidal properties are particularly sharply developed in those oxides (Al_{2}O_{3}, SiO_{2}, MoO_{3}, SnO_{2}, &c.) which show (like water also) the properties of feeble bases and feeble acids, there is probably some causal reason for this coincidence, all the more so since among organic substances--gelatins, albumins, &c.--the representatives of the colloids also have the property of feebly combining with bases and acids. [30] Since Deville's experiments the question of the density of aluminium chloride has been frequently re-investigated. The subject has more especially occupied the attention of Nilson, Pettersson, Friedel and Crafts, and V. Meyer and his collaborators. In general, it has been found that at low temperatures (up to 440°) the density is constant, and indicates a molecule Al_{2}Cl_{6}; whilst depolymerisation probably (although it is not yet certain) takes place at higher temperatures, and the molecule AlCl_{3} is obtained. Along with this there has been, and still is, a difference of opinion as to the vapour density of aluminium ethyl and methyl--whether for instance, Al(CH_{3})_{3} or Al_{2}(CH_{3})_{6} expresses the molecule of the latter. The interest of these researches is intimately connected with the question of the valency of aluminium, if we hold to the opinion that elements in their various compounds have a constant and strictly definite valency. In this case the formula AlCl_{3} or Al(CH_{3})_{3} would show that Al is trivalent, and that consequently the compounds of aluminium are Al(OH)_{3}, AlO_{3}Al, and, in general, AlX_{3}. But if the molecule be Al_{2}Cl_{6}, it is--for the followers of the doctrine of the invariable valency of the elements--incompatible with the idea of the trivalency of aluminium, and they assume it to be quadrivalent like carbon, likening Al_{2}Cl_{6} to ethane C_{2}H_{6} = CH_{3}CH_{3}, although this does not explain why Al does not form AlCl_{4}, or, in general, AlX_{4}. In this work another supposition is introduced; according to this, although aluminium, as an element of group III., gives compounds of the type AlX_{5}, this does not exclude the possibility of these molecules combining with others, and consequently with _each other_--that is, forming Al_{2}X_{6}; just as the molecules of univalent elements exist either as H_{2}, Cl_{2}, &c., or as Na, and the molecules of bivalent elements either as Zn, or as S_{2}, or even S_{6}. In the first place it must be recognised that the limiting form does not exhaust all power of combination, it only exhausts the capacity of the element for combining with X's, but the saturated substance may afterwards