Note 35 — Chapter XVII · BORON, ALUMINIUM, AND THE ANALOGOUS METALS OF THE THIRD GROUP

Here we see an instance in confirmation of what has been said in Note 30—i.e. the action of the molecule AlCl3. 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 (Al2O3)6(CaO)10CaCl2. Even clay forms a similar stony substance, which might be of practical use.

Among the most complex compounds of aluminium, ultramarine, or lapis lazuli, must be mentioned. It occurs in nature near Lake Baikal, in crystals, some colourless and others of various tints—green, blue, and violet. When heated it becomes dull and acquires a very brilliant blue colour. In this form it is used for ornaments (like malachite), and as a brilliant blue pigment. At the present time ultramarine is prepared artificially in large quantities, and this process is one of the most important conquests of science; for the blue tint of ultramarine has been the object of many scientific researches, which have culminated in the manufacture of this native substance. The most characteristic property of ultramarine is that when placed in sulphuric acid it evolves hydrogen sulphide and becomes colourless. This shows that the blue colour of ultramarine is due to the presence of sulphides. If clay be heated in a furnace with sodium sulphate and charcoal (forming sodium sulphide) without access of air, a white mass is obtained, which becomes green when heated in the air, and when treated with water leaves a colourless substance known as ‘white ultramarine.’ When ignited in the air it absorbs oxygen and turns blue. The coloration is ascribed to the presence of metallic sulphides or polysulphides, but it is most probable that silicon sulphide, or its oxysulphide, SiOS, is present. At all events the sulphides play an important part, but the problem is not yet quite settled. The formula Na₈Al₆Si₆O₂₄S is ascribed to white ultramarine. The green probably contains more sulphur, and the blue a still larger quantity. The last is supposed to contain Na₈Al₆Si₆O₂₄S₃. It is more probable (according to Guckelberger, 1882) that the composition of the blue varies between Si₁₈Al₁₈Na₂₀S₆O₇₁ and Si₁₈Al₁₂Na₂₀S₆O₆₉. The latter may be expressed as (Al₂O₃)₆(SiO₂)₁₈(Na₂O)₁₀S₆O₅, which would indicate the presence of insufficiently-oxidised sulphur in ultramarine.