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.
[26] Compounds of alumina with bases (aluminates, see Note [21]) are sometimes met with in nature. Such are spinel (see p. [65]), MgO,Al2O3 = MgAl2O4, chrysoberyl, BeAl2O4, and others. Magnetic oxide of iron, FeO,Fe2O3 = Fe3O4, and compounds like it, belong to the same class. Here we evidently have a case of combination ‘by analogy,’ as in solutions and alloys, accompanied by the formation of strictly definite saline compounds, and such instances form a clear transition from so-called solutions and certain mixtures to the type of true salts.
[27] Not only aluminium acetate (Note [24]), but also every other aluminium salt with a volatile acid, parts with its acid on heating an aqueous solution—that is, is decomposed by water, and forms either basic salts or a hydrate of alumina. By dissolving aluminium hydroxide in nitric acid we may easily obtain a well-crystallising aluminium nitrate, Al(NO3)3,9H2O, which fuses at 73° without decomposing (Ordway), gives a basic salt, 2Al2O3,6HNO3, at 100°, and at 140° leaves the aluminium hydroxide perfectly free from the elements of nitric acid. But the solutions of this salt, like those of the acetate, are also able to yield aluminium hydroxide. From all this it is evident that we must suppose that the solutions of this and similar salts contain an equilibrated dissociated system, containing the salt, the acid, and the base, and their compounds with water, as well as partly the molecules of water itself. Such examples much more clearly confirm those conceptions of solutions which are given in the first chapter than a general preliminary acquaintance with the subject can do.
[28] As an example of native basic salts we may cite alunite, or alum-stone (sp. gr. 2·6), which sometimes occurs in crystals, but more frequently in fibrous masses. It has been found in masses in the Caucasus (at Zaglik, forty versts distance from Elizabetpol), and at Tolfa, near Rome. Its composition is K2O,3Al2O3,4SO3,6H2O (alunite contains 9H2O). It is soluble in water but not decomposed by it, but after being slightly ignited it gives up alum to it. It may be artificially prepared by heating a mixture of alum with aluminium sulphate in a closed tube at 230°.
[29] As the colloidal properties are particularly sharply developed in those oxides (Al2O3, SiO2, MoO3, SnO2, &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 Al2Cl6; whilst depolymerisation probably (although it is not yet certain) takes place at higher temperatures, and the molecule AlCl3 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(CH3)3 or Al2(CH3)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 AlCl3 or Al(CH3)3 would show that Al is trivalent, and that consequently the compounds of aluminium are Al(OH)3, AlO3Al, and, in general, AlX3. But if the molecule be Al2Cl6, 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 Al2Cl6 to ethane C2H6 = CH3CH3, although this does not explain why Al does not form AlCl4, or, in general, AlX4. In this work another supposition is introduced; according to this, although aluminium, as an element of group III., gives compounds of the type AlX5, this does not exclude the possibility of these molecules combining with others, and consequently with each other—that is, forming Al2X6; just as the molecules of univalent elements exist either as H2, Cl2, &c., or as Na, and the molecules of bivalent elements either as Zn, or as S2, or even S6. 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 combine with whole molecules, which fact is best proved by the capacity of substances to form crystalline compounds with water, ammonia, &c. But in some substances this faculty for further combinations is less developed (for instance, in carbon tetrachloride, CCl4), whilst in others it is more so. AlX3 combines with many other molecules. Now if a limiting form, which does not combine with new X's, nevertheless combines with other whole molecules, it will naturally in some instances combine with itself, will polymerise. In this manner the mind clearly grasps the idea that the same forces which cause S2 to unite itself to Cl2, or C2H4 to Cl2, &c., also unite molecules of a similar kind together; thus polymerisation ceases to be an isolated fragmentary phenomenon, and chemical combinations ‘by analogy’ acquire a particular and important interest. In conformity with these views the following proposition may be made concerning the compounds of aluminium. They are of the type AlX3 in the limit, like BX3, but those limiting forms are still able to combine to form AlX3,RZ, and the aluminium chloride is a compound of this kind—i.e. (AlX3)2. In boron, for example, in BCl3, this tendency to form further compounds is less developed. Hence boron chloride appears as BCl3, and not (BCl3)2. Polymerisation is not only possible when a substance has not attained the limit (although it is more probable then), but also when the limiting form has been reached, if only the latter has the faculty of combining with other whole molecules. We may therefore conclude that aluminium, like boron, is trivalent in the same sense that lithium and sodium are univalent, magnesium bivalent, and carbon tetravalent. In a word, there is no reason to consider that aluminium is capable of forming compounds AlX4, and in that way to explain the existence of the molecule Al2Cl6. Furthermore, there are many reasons for thinking that AlF3, Al2O3, and other empirical formulæ do not express the molecular weights of these compounds, but that they are much higher: AlnF3n, Al2nO3n. In recent years convincing proofs of the truth of the above statements have been obtained, and of the independent existence of AlX3 in a state of vapour; for Comb has determined the vapour density of the volatile acetyl of aluminium acetate Al(C3H7O2)3 (which melts at 193°, boils at 315°, and distils without a trace of decomposition), and has found that it exactly corresponds to the above molecular composition. On the other hand, Louise and Roux (1889) by employing the method of ‘freezing point depression’ of solutions (Chapter I., Note [49]) found that the molecules Al2(C2H5)6 and Al2(C5H11)6, &c., correspond to the type Al2X6. Thus it may now be accepted that the molecular composition of the compounds of aluminium in their simplest form is AlX3, but that they may polymerise and give Al2X6 or, in general, Al2X3n.
[31] In the case of gallium, as a close analogue of aluminium, Lecoq de Boisbaudran (1880) showed that probably the molecule gallium chloride contains Ga2Cl6 at low temperatures and high pressures, and that it dissociates into GaCl3 at high temperatures and low pressures. The molecule of indium chloride seems to exist only in the simplest form, InCl3.
[32] The pure salt (16H2O) is not hygroscopic. In the presence of impurities the amount of water increases to 18H2O, and the salt becomes hygroscopic.
[33] The common form of crystals of alums is octahedral, but if this solution contains a certain small excess of alumina above the ratio 2Al(OH)3 to K2SO4, and not more sulphuric acid than 3H2SO4 to 2Al(OH)3, then it easily forms combinations of the cube and octahedron, and these alums are called ‘cubic’ alums. They are valued by the dyer because they can contain no iron in solution, for oxide of iron is precipitated before alumina, and if the latter be in excess there can be no oxide of iron present. These alums were long exported from Italy, where they were prepared from alunite (Note [28]).
[33 bis] It is also formed by the action of hydrochloric acid upon metallic aluminium (Nilson and Pettersson), by heating alumina in a mixture of the vapours of naphthaline and HCl (Faure, 1889), and by the action of dry HCl upon an alloy of 14 p.c., or more of Al and copper (Mobery).