[26] Compounds of alumina with bases (aluminates, _see_ Note 21) are sometimes met with in nature. Such are spinel (_see_ p. 65), MgO,Al_{2}O_{3} = MgAl_{2}O_{4}, chrysoberyl, BeAl_{2}O_{4}, and others. Magnetic oxide of iron, FeO,Fe_{2}O_{3} = Fe_{3}O_{4}, 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(NO_{3})_{3},9H_{2}O, which fuses at 73° without decomposing (Ordway), gives a basic salt, 2Al_{2}O_{3},6HNO_{3}, 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 K_{2}O,3Al_{2}O_{3},4SO_{3},6H_{2}O (alunite contains 9H_{2}O). 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°.