amount of sulphuric acid and its salts will cause the alumina to gelatinise--that is, cause the hydrogel to separate. Many such colloidal solutions are known (Vol. I. p. 98, Note 57). [25] In a dialyser, Vol. I. p. 63, Note 18. [25 bis] The different states in which the hydrates of alumina occur and are prepared resemble similar varieties of the hydrates of the oxides of iron and chromium, of molybdic and tungstic acids, as well as of phosphoric and silicic acids, of many sulphides, proteid substances, &c. We shall therefore have occasion to recur to this subject in the further course of this work. The most remarkable peculiarity of Graham's solution is that it solidifies on litmus paper, and leaves a blue ring on it, which shows the alkaline--that is, basic--character of the alumina in such a solution. If in the dialysis the basic hydrochloric acid salt be replaced by a similar acetic acid salt, a hydrosol of alumina is obtained which does not act upon litmus. With respect to alumina as a base, it is very important to observe that it is not only capable of combining with other bases[26] but that it does not give salts with feeble volatile acids (like carbonic and hypochlorous); it forms salts which are easily decomposed by water, especially when heated,[27] as well as double and basic salts,[28] _so that it forms a clear example of a feeble base_.[29] To these characteristics of alumina we must add that it not only gives compounds of the type AlX_{3}, but also the polymeric type Al_{2}X_{6}, even when X is a simple univalent haloid like chlorine. Deville and Troost showed (1857) that the vapour density of aluminium chloride (at about 400°) is 9·37 with respect to air--that is, nearly 135 with respect to hydrogen, and therefore the formula of its molecule is expressed by Al_{2}Cl_{6}, and not AlCl_{3},[30] although in the case of boron, arsenic, and antimony, which give oxides R_{2}O_{3} of the same composition as Al_{2}O_{3}, the chlorine compounds form non-polymeric molecules, BCl_{3}, AsCl_{3}, SbCl_{3}.[31] This duplication (polymerisation) of the form AlX_{3} is connected with the facility with which the salts of aluminium combine with other salts to form double salts and with aluminium hydroxide itself to form basic salts.