BOOK XIV.

CHEMISTRY.

A

CHAPTER IX.

THE ELECTRO-CHEMICAL THEORY.

MONG the consequences of the Electro-chemical Theory, must be ranged the various improvements which have been made in the voltaic battery. Daniel introduced between the two metals a partition permeable by chemical action, but such as to allow of two different acid solutions being in contact with the two metals. Mr. Grove's battery, in which the partition is of porous porcelain, and the metals are platinum and amalgamated zinc, is one of the most powerful hitherto known. Another has been constructed by Dr. Callan, in which the negative or conducting plate is a cylinder of cast iron, and the positive element a cylinder of amalgamated zinc placed in a porous cell. This also has great energy.

The Number of Elementary Substances.

Pelo

There have not been, I believe, any well-established additions to the list of the simple substances recognized by chemists. Indeed the tendency at present appears to be rather to deny the separate elementary character of some already announced as such substances. pium and Niobium were, as I have said, two of the new metals. But Naumann, in his Elemente der Mineralogie (4th ed. 1855), says, in a foot note (page 25): 'Pelopium is happily again got rid of; for Pelopic Acid and Niobic Acid possess the same Radical. Donarium had a still shorter existence.'

In the same way, when Hermann imagined that he

had discovered a new simple metallic substance in the mineral Samarskite from Miask, the discovery was disproved by H. Rose (Pogg. Ann. B. 73, s. 449).

In general the insulation of the new simple substances, the metallic bases of the earths, and the like,— their separation from their combinations, and the exhibition of them in a metallic form-has been a difficult chemical process, and has rarely been executed on any considerable scale. But in the case of Aluminium, the basis of the earth Alumina, the process of its extraction has recently been so much facilitated, that the metal can be produced in abundance. This being the case, it will probably soon be applied to special economical uses, for which it is fitted by possessing special properties.

BOOK XV.

MINERALOGY.

B

Y the kindness of W. H. Miller, Esq., Professor of Mineralogy in the University of Cambridge, I am able to add to this part the following notices of books and memoirs.

1. Crystallography.

Elemente der Krystallographie, nebst einer tabellarischen Uebersicht der Mineralien nach der Krystallformen, von Gustav Rose. 2. Auflage. Berlin, 1838. The crystallographic method here adopted is, for the most part, that of Weiss. The method of this work has been followed in

A System of Crystallography, with its Applications to Mineralogy. By John Joseph Griffin. Glasgow, 1841. Mr. Griffin has, however, modified the notation of Rose. He has constructed a series of models of crystalline forms.

1842. This

Frankenheim's System der Krystalle. work adopts nearly the Mohsian systems of crystallization. It contains Tables of the chemical constitution, inclinations of the axes, and magnitude of the axes of all the crystals of which a description was to be found, including those formed in the laboratory, as well as those usually called minerals; 713 in all.

Fr. Aug. Quenstedt, Methode der Krystallographie, 1840, employs a fanciful method of representing a crystal by projecting upon one face of the crystal all the other faces. This invention appears to be more curious than useful.

Dr. Karl Naumann, who is spoken of in Chap. IX. of this Book, as the author of the best of the Mixed Systems of Classification, published also Grundriss der Krystallographie. Leipzig, 1826. In this and

other works he modifies the notation of Mohs in a very advantageous manner.

Professor Dana, in his System of Mineralogy, New Haven (U.S.), 1837, follows Naumann for the most part, both in crystallography and in mineral classification. In the latter part of the subject, he has made the attempt, which in all cases is a source of confusion and of failure, to introduce a whole system of new names of the members of his classification.

The geometry of crystallography has been investigated in a very original manner by M. Bravais, in papers published in the Journal of the Ecole Polytechnique, entitled Mémoires sur les Systèmes formés par des Points. 1850. Etudes Crystallographiques. 1851.

Hermann Kopp (Einleitung in die Krystallographie, Braunschweig, 1849) has given the description and measurement of the angles of a large number of laboratory crystals.

Rammelsberg (Krystallographische Chemie, Berlin, 1855) has collected an account of the systems, simple forms and angles of all the laboratory crystals of which he could obtain descriptions.

Schabus of Vienna (Bestimmung der Krystallgestalten in Chemischen Laboratorien erzeugten Producte, Wien, 1855; a successful Prize Essay) has given a description, accompanied by measurements, of 90 crystalline species from his own observations.

To these attempts made in other countries to simplify and improve crystallography, I may add a remarkable Essay very recently made here by Mr. Brooke, and suggested to him by his exact and familiar knowledge of Mineralogy. It is to this effect. All the crystalline forms of any given mineral species are derived from the primitive form of that species; and the degree of symmetry, and the parameters, of this form determine the angles of all derivative forms. But how is this primitive form selected and its parameters determined? The selection of the kind of the primitive form depends upon the degree of symmetry which appears in all the derivative forms; according to which they belong to the rhombohedral, prismatic, square

pyramidal, or some other system: and this determination is commonly clear. But the parameters, or the angles, of the primitive form, are commonly determined by the cleavage of the mineral. Is this a sufficient and necessary ground of such determination? May not a simplification be effected, in some cases, by taking some other parameters? by taking a primitive form which belongs to the proper system, but which has some other angles than those given by cleavage? Mr. Brooke has tried whether, for instance, crystals of the rhombohedral system may not be referred with advantage to primitive rhombohedrons which have, in all the species, nearly the same angles. The advantage to be obtained by such a change would be the simplification of the laws of derivation in the derivative forms: and therefore we have to ask, whether the indices of derivation are smaller numbers in this way or with the hitherto accepted fundamental angles. It appears to me, from the examples given, that the advantage of simplicity in the indices is on the side of the old system: but whether this be so or not, it was a great benefit to crystallography to have the two methods compared. Mr. Brooke's Essay is a Memoir presented to the Royal Society in 1856.

2. Optical Properties of Minerals.

The Handbuch der Optik, von F. W. G. Radicke, Berlin, 1839, contains a chapter on the optical properties of crystals. The author's chief authority is Sir D. Brewster, as might be expected.

M. Haidinger has devoted much attention to experiments on the pleochroism of minerals. He has invented an instrument which makes the dichroism of minerals more evident by exhibiting the two colours side by side.

The pleochroism of minerals, and especially the remarkable clouds that in the cases of Iolite, Andalusite, Augite, Epidote, and Axinite, border the positions of either optical axis, have been most successfully imitated