into several simple forms by different solutions and successive slow crystallizations. Thus cubo-octo-dodecahedral alum yielded separate octahedrons, cubes, and cubo-dodecahedrons.

16. Crystals of a certain form being put into a solution of the same substance, which gives naturally a different form, increase by additions according to this new form.-(See Ann. de Chim. et Phys. viii. 5.)

V. ELECTRICITY OF MINERALS.

M. Haüy, assisted by M. Delafosse, has made a very elaborate set of experiments to determine the electrical state of the different species of minerals. I can here give nothing more than a tabular view of the results which they obtained. This, indeed, is sufficient, as the mode of trying the electricity of minerals has been long familiar to mineralogists.

CLASS I.

Substances transparent and colourless in their perfect state. Their colour, when they have any, depends upon an accidental principle. They are capable of insulating, and acquire, when rubbed, the vitreous or positive electricity.

The following species are placed here merely from analogy:

Substances exhibiting resinous or negative electricity joined to an unctuous feel. They are capable of insulating when transparent and colourless.

Foliated talc,

Granular talc ?

CLASS II.

Agalmátalmolite?!

Substances having a peculiar colour depending on their nature, capable of insulating in what state soever they are, and acquiring, when rubbed, resinous or negative electricity. Anthracite alone must be insulated before it can be excited.

Sulphur,

Bitumen,

a. glutinous,

b. solid,

c. elastic,

Retinasphalt,
Amber,
Mellite,
Anthracite.

CLASS III.

Substances essentially opaque, possessing the metallic lustre, or acquiring it when polished, conductors, and acquiring when insulated, some of them vitreous, and others resinous electricity.

B. Exhibiting only a tendency to the Metallic Lustre, which they acquire sensibly when polished.

Substances having a colour, depending on their nature, susceptible of transparency in their perfect state. The property of insulating is limited to those varieties which approach that

state.

ORDER 1.

Susceptible of giving by reflexion the metallic lustre, and by reflection and refraction at once a colour more or less lively. The difference depends on the polish of the surface. They all acquire resinous electricity by friction,

Destitute of the Metallic Lustre. Almost all acquire Resinous

Electricity when rubbed.

This historical sketch has been insensibly carried to such a length, that I am deprived of the power of entering into those geological details which the popularity of the science, and the zeal with which it has been cultivated in Great Britain, and in some other countries, would have rendered both amusing and instructive. I regret this preclusion the less, because the most

important facts which have come to my knowledge, either have or will make their appearance in the transactions of the different geological societies which have been of late years established in Great Britain. I shall take care to insert a regular analysis of the different volumes published by these societies into the Annals of Philosophy soon after they have made their appearance.

There is only one publication belonging to Geology, strictly so called, which has made its appearance since my last historical sketch was drawn up. I allude to a work, intitled, "Facts and Observations towards forming a new Theory of the Earth, by William Knight, LL. D. Professor of Natural Philosophy in the Institution of Belfast." I abstain the more willingly from entering into any discussion respecting the theory of the earth, which the author has advanced, and which he has supported with much zeal and ingenuity, because the world in general seems now sensible of the unprofitable nature of such speculations. Even Professor Jameson, whose zeal burned for so many years with such furious ardour, that to call in question a Wernerian opinion, or to hesitate about the propriety of a Wernerian arrangement, was considered by him as a crime of the deepest die, and worthy of the severest treatment; even he has become sufficiently cool, has ventured to call in question some of the most material parts of his master's geognosy; and if he exercise his own judgment without fetters for a few years longer, I venture to predict that he will not be a Wernerian at all. Even the Huttonians, those Calvinists of the science of geology, whose theory was so complete and so beautiful, if we took its foundation for granted, and were complaisant enough to overlook its inconsistency with the phenomena of nature-even they have become a great deal more tolerant; they no longer hurl their anathemas and their interdicts against their antagonists; they no longer affirm that mineralogy and geology are unconnected sciences, and that we may become profound geologists without any knowledge whatever of rocks or of minerals. On the contrary, they have exercised their industry with laudable zeal, and not only favoured us with descriptions of tracts of country themselves, but encouraged others to undertake similar tasks. Geologists in general seem now satisfied that the true object of their science is to acquire an accurate knowledge of the structure of the earth; that this knowledge can be acquired only by patient observation; that at present our knowledge of that structure is very incomplete; and that till the position of all the different strata over the whole surface of the earth be accurately ascertained, it would be a waste of time to speculate upon the original formation of these strata, or the changes which they have undergone since their original creation. Dr. Knight is a gentleman of amiable manners, of excellent abilities, and indefatigable industry. He would much more effectually pro