All these halogen compounds of aluminium are soluble in water. _Aluminium fluoride_, AlF_{3} (Al_{_n_}F_{3_n_}), is insoluble in water. It is obtained by dissolving alumina in hydrofluoric acid; a solution is then formed, but it contains an excess of hydrofluoric acid. When this solution is evaporated, crystals containing Al_{2}F_{6},HF,H_{2}O are obtained. They are also insoluble in water. By saturating the above solution with a large quantity of alumina, and then evaporating, we obtain crystals having the composition Al_{2}F_{6},7H_{2}O. All these compounds, when ignited, leave insoluble anhydrous aluminium fluoride. It forms colourless rhombohedra, which are non-volatile, of sp. gr. 3·1, and are decomposed by steam into alumina and hydrofluoric acid. The acid solution apparently contains a compound which has its corresponding salts; by the addition of a solution of potassium fluoride, a gelatinous precipitate of AlK_{3}F_{6} is obtained. A similar compound occurs in nature--namely, AlNa_{3}F_{6}, or _cryolite_, sp. gr. 3·0. [34 bis] In this respect aluminium chloride resembles the chloranhydrides of the acids, and probably in the aqueous solution the elements of the hydrochloric acid are already separated, at least partially, from the aluminium hydroxide. The solution may also be obtained by the action of aluminium hydroxide on hydrochloric acid. [35] Here we see an instance in confirmation of what has been said in Note 30--_i.e._ the action of the molecule AlCl_{3}. We will cite still another instance confirming the power of alumina to enter into complex combinations. Alumina, moistened with a solution of calcium chloride, gives, when ignited, an anhydrous crystalline substance (tetrahedral), which is soluble in acids, and contains (Al_{2}O_{3})_{6}(CaO)_{10}CaCl_{2}. Even clay forms a similar stony substance, which might be of practical use.