the earth takes its name. It is usually procured by lixiviating certain sulphureous argillaceous slaty rocks, on which alum is naturally disposed to effloresce. The function of alumina, in these, and other salts and minerals into which it enters, is very analogous to that of water. The salts remain acidous; and, in the case of all the alums, of which there are several species, it is necessary to afford a nucleus for the crystalline molecule of some other salt, such as sulphate of potash, soda, iron, or ammonia. Around this, according to the number proper to the ternate acid, are three of vitriolate of alumina. The most common sort of alum is that in which one of sulphate of potash is associated with three of vitriolate of alumina, each of the particles of vitriolate of alumina bearing six particles of water, and one more, essential to the vitriol; further, there are probably three particles of water on the poles of the vitriols in union with the alumina, common to two such semimolecules which are thereby united into one symmetrical body; and two particles of water more are on each pole of the symmetrical molecule. The investigation of such molecules, however, requires the aid of crystallography: they all give rise to a regular octaedron. According to this view, there are on one side of the equator of such a molecule three particles of alumina 72, one of potash 60, four of acid 200, and twentythree and a half particles of water. Its real structure, how

Besides this substance, in which one particle of alumina is on the pole of one particle of oil of vitriol, others are found in which the alumina takes the place of water. Thus there is an aluminous salt, composed of a particle of oil of vitriol with one of alumina on each pole, and a third, in which there are three particles of alumina to one of acid.

One particle of the former seems to serve as a nucleus for three of the latter, in the constitution of the mineral named Aluminite, 34 particles of water being occupied in the molecule*. The subsulphate of alumina and potash of Riffault seems also to consist of three particles of the ternialuminous sulphate; but the nucleus, instead of being a particle of the dialuminous sulphate, as in aluminite, is a particle of sulphate of potash; and the same analysis is applied to the alumstone from Solfa, near Civita Vecchia, used in the manufacture of alum (Mohs' Mineralogy, vol. ii. p. 68), though, in the cimen analyzed by Vauquelin, silex entered largely.

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Phosphate of Alumina.-Phosphoric acid is another ternate acid, and alumina occurs in nature united to it, constituting the rare mineral named Wavellite. The specimen analyzed by Fuchs contained only alumina, phosphoric acid, and water, and the structure is very explicitly indicated by his analysis. The alumina, as usual, occupies the place of water. particle forms the nucleus for a distribution of phosphoric acid, similar to that of the glacial acid, that is, a particle of alumina forms the nucleus, and three others cover the naked poles of the three particles of phosphoric acid, attached to the equatorial region of the central alumina; and besides these five particles of phosphoric acid 90, and four of alumina 96, there are six of water 72.

Acetate of Alumina.-It is evident that three particles of acetic acid, attached to their particle of water, may be accom

modated on the pole of a particle of alumina in a most symmetrical manner. But the number of acetates which might be generated by mixing the acid and base in due proportions, seems to be very great. A particle of alumina is capable of distributing symmetrically about it, almost any number of acetic particles up to fourteen, and Wenzel seems to have examined an acetate of the latter sort. That which consists of a ternate molecule of acetic acid and a particle of alumina, crystallizes in prisms, which adhere very firmly to the aluminous dish in which they form.

Muriate of Alumina.—Muriatic acid forms, with alumina, solid matter, very different from the chlorides, and not capable, by the ordinary methods, of being converted into a chloride. There are two muriates, one consisting of a particle of acid, with one of base and three of water; the other, analyzed by Bucholz, consisting of one of acid with five of water, which is the number proper to the acid when its pole is covered, united to two of alumina. There are many other substances which may be formed in the laboratory by the union of alumina and acids, but they are of little interest. Alumina is too weak a base for procuring very interesting results in the rapid and rude treatment of the laboratory. It is to nature that we must look for the highly symmetrical forms, of which alumina constitutes a part.

Aluminic Minerals.—Alumina and silex are, in the crystalline world, what hydrogen and carbon are in the vegetable. The number of their combinations seems to be as great as the specific forms to which they give rise are numerous. Although, as in the vegetable kingdom, there may be certain molecules, as, for instance, a double molecule of silica, which are introduced in great numbers in constructing crystalline forms; yet here and there, as fulcra and nuclei, single par

ticles of one or other are, no doubt, introduced, and to detect the true chemical constitution of the great majority of mineral species, will probably require not only the most rigid and unprejudiced analysis, but all the aid of a crystallographic synthesis.

There are many, however, of the more notable crystalline bodies, which even the analyses now extant enable us to perceive that they possess a very simple structure. Thus that interesting mineral the Petrosilex* of Sahlberg in Sweden, indicates, by the analysis of Berthier, a structure analogous to that of Hyalite, the water being replaced by alumina.

Felspar †, again, consists of one particle of silex and one of alumina, with one of potash to every four. Albite has the same composition, but the potash is replaced by soda, and stilbite seems to agree with them, the potash and soda of the others being replaced by lime.

In Analcime, again, the ratio of the earthy constituents are the same, but there is one of soda to every three particles. In some other minerals there is a double molecule of silex. Thus, Leucite has a double molecule of silica, three of alumina, and a fourth of potassa.

In Meionite there is a single particle of silica with the same quaternate arrangement around it, three parts being alumina, and the fourth a particle of lime. But until the methods of crystalline analysis be improved, these views need not be extended.

WE may resume the subject of metallic forms to illustrate the properties of two, which have been known from a very remote antiquity, and are still more extensively used in the arts than any other. These are Copper and Iron, the former of which receives additional interest from its utility in the docimastic art, while the latter seems to hold a place in nature inferior in interest and importance only to the radiant matter itself. Circumstances, afterwards to be mentioned, render it extremely probable that copper is a form composed of four atoms, inclosing a tetraedral cavity, with two particles of hydrogen applied to the two opposite re-entrant angles (Fig. 89). Its atomic weight is 8. But its form is much less parasitic than any of the other metallic bodies which have yet been noticed, and it may be preserved in the air in the metallic state. If we arrange these particles symmetrically one upon another, we shall find that filaments are produced; hence copper is a ductile metal. There is also an arrangement of edges very similar to that which obtains in sulphur, which may be connected with the similarity of their colours. But copper is also capable of giving rise to symmetrical quinate molecules (Fig. 90), whose atomic weight is 40; and hence it is sometimes met with in the ordinary tessular forms of crystallization. Native