ON NATIVE HYDROUS ALUMINATE OF LEAD, OR PLOMB GOMME.

From Thomson's Annals of Philosophy, Vol. XIV, 1819, p. 81.

PARIS, May 22, 1819.

I see in the Annals of Philosophy for this month, which I have very lately received, an analysis by M. Berzelius of the mineral which was formerly known here under the name of "plomb gomme."

The first discovery of the composition of this singular substance belongs, however, to my illustrious and unfortunate friend, and indeed distant relative, the late Smithson Tennant. He ascertained when last at Paris, on pieces furnished him by M. Gillet de Laumont, that it was a combination of oxide of lead, alumina, and water.

At that time I received a small specimen of this rare ore from M. de Laumont, accompanied with a label, of which the following is a copy:

"Hydrate d'alumine et de plomb reconnu par Mr. Tennant, du Huelgoat, près Poullaouen, en Bretange (Finisterre) qui paroit etre la même substance decrite par Romé de l'Isle, tom. iii. de la Cristallographie, p. 399, comme plomb rouge en stalactite.

"J'en ai dit quelques mots en Mai, 1786, dans le Journal de Physique, p. 385, F. 16."

This ore is of a yellow colour; it otherwise bears so great a resemblance to the siliceous substance found near Frankfort on the Mein, called Müllen glass, that it might be mistaken for it.

Suddenly heated, it decrepitated violently; but heated. slowly, it became white and opaque. The utmost fire did not appear to fuse it, or produce any further alteration in it.

It dissolved readily in borax into a colourless transparent glass, but no reduction of lead took place. Not having any

carbonate of soda at hand, I added a particle of nitre, whose deflagration producing potash, lead was revived.

A bit, which had been made white by ignition, being wetted with nitrate of cobalt and again ignited, became blue.

Heated in a glass tube over a candle, it decrepitated, became opaque and white, and water sublimed.

Mr. Tennant mentioned to me a sort of explosion occasioned by the sudden expulsion of the water, and characteristic of this ore, which took place when it was heated at the blow-pipe. With the very minute particles I have tried, no effect of this sort was perceived.

The above characters will prove sufficient, I apprehend, to make this substance known when met with.

From Thomson's Annals of Philosophy, Vol. XVI, 1820, p. 100. Plomb Gomme.-Mr. Smithson has given us some interesting details respecting the history and properties of this mineral, which is a hydrous aluminate of lead. It has a yellow colour, and is exceedingly similar in appearance to Mullen glass. When heated, it decrepitates violently; and if it be heated by the blow-pipe, in contact with an alkali, lead is reduced. Its nature was first ascertained by Mr. Tennant. Berzelius has lately analyzed it. The result of his analysis will be found in the Annals of Philosophy, xiii. 381. (See Annals of Philosophy, xiv. 31.)

ON A FIBROUS METALLIC COPPER.

From Thomson's Annals of Philosophy, Vol. XVI, 1820, p. 46.

PARIS, March 17, 1820.

SIR: There occur, in mineral collections, pieces of a copper slag, having fibres of metallic copper in its cavities. I have seen this fibrous copper erroneously placed among native coppers.

I possess samples of this kind from a foundery in the Hartz. The metallic copper in the cavities, or air-holes, is so delicately slender as to be a metallic wool.

From several considerations, it appeared to me to be beyond all doubt that the opinion of these fibres having been produced by crystallization was perfectly inadmissible; and I was for a very long time totally unable to come to any conjecture with respect to the mode in which they had originated.

Looking on one of these specimens this morning, an idea struck me which is, I am convinced, the solution of this knotty problem.

It occurred to me that these fibres had been generated at the instant of consolidation of the fused slag. That by its shrinking at that moment, it had compressed drops of copper, still in a fluid state, dispersed in its substance, and squeezed a portion of it through the minute spaces between its particles, under this fibrous form, into its cavities, or airholes.

For this operation to take place, the concurrence of several conditions is required. The slag must be so thick and pasty as to retain metallic copper scattered through it. It must have developed bubbles of some gas which have occasioned vacuities in it. It must be less fusible than the copper, but in so very small a degree that the copper consolidates as the fibres of it are formed.

It is evident that on this supposition these fibres of copper are produced by a process entirely the same as that employed for the manufactory of macaroni and vermicelli; and which are made by forcing paste through small apertures by the pressure of a syringe. It is wire-drawing performed inversely-by propulsion instead of traction.

As soon as this hypothesis had presented itself to me, I became anxious to ascertain whether I could give birth to this fibrous copper at the blow-pipe. I melted a small fragment of the slag; and, on breaking it, I had the gratification of finding its little cavities lined with minute fibres of metallic copper as those of its greater prototype.

I wished now to form the slag itself which was to afford the copper fibres. As I had ascertained the slag of the

Hartz to consist of sulphur, copper, and iron, I had recourse to the yellow sulphuret of copper and iron. To produce the required portion of metallic copper, I calcined some small fragments of this yellow ore at the tip of the exterior flame. Finding that I had exceeded the proper point, and rendered them too infusible, I added a little of the raw ore; and after encountering a few difficulties succeeded in producing a little mass of slag, whose internal cavities presented me, on breaking it, with the fibres of copper which were the object of my toil.

A repetition of these experiments in a furnace, on a larger scale, would undoubtedly have yet more successful results.

It deserves to be noticed that the curved form which these fibres of copper generally have is entirely favourable to the foregoing theory of their formation, and equally contrary to the supposition of their being produced by crystallization.

The power to which has been ascribed the phenomenon which forms the subject of these pages has hitherto been overlooked. It has not been considered what the effects might be of the contraction of a melted mass at the moment of its congelation. It is, however, a means of effects which may have acted on many occasions in the earth. Two matters of unequal fusibility, and of no attraction to each other, are not unlikely to have occurred blended in a state of fusion; and then the most fusible to have become pressed out from between the particles of the other when it solidified. If some evolved vapour had opened cavities in the mass, or rents had formed in it, the fluid matter will have escaped from the pressure into these voids, as has happened with the copper. If these receptacles for it have been wanting, it must have flowed to the external surfaces, and may have formed a crust there. The matter which lines or fills the cavities of some lavas has, perhaps, been so introduced into them.

A knowledge of the productions of art, and of its operations, is indispensable to the geologist. Bold is the man who undertakes to assign effects to agents with which he

has no acquaintance; which he never has beheld in action; to whose indisputable results he is an utter stranger; who engages in the fabrication of a world alike unskilled in the forces and the materials which he employs.

AN ACCOUNT OF A NATIVE COMBINATION OF SULPHATE OF BARIUM AND FLUORIDE OF CALCIUM.

From Thomson's Annals of Philosophy, Vol. XVI, 1820, p. 48.

PARIS, March 24, 1820. SIR: I acquired this substance in Derbyshire. It is many years since I ascertained its constitution. I have examined several minerals which in appearance bore a resemblance to it, but have not found any of them to be of the same nature. This species would hence appear to be of rare occurrence in the earth.

This substance formed a vein about an inch wide in a coarse shell limestone. Next to this substance was a layer of crystals of sulphuret of lead; and between these and the limestone rock a layer of crystals of carbonate of calcium.

I infer that these matters filled a vertical fissure in the limestone stratum; and from the ideas I entertain of the mode by which such fissures have generally become occupied by their contents, I believe them to have been successively deposited in it by sublimation, either through the intense vehemence of subterranean fire, or by the agency of the vapour of water, or of some other gas.

This compound matter bears in its general appearance so strong a resemblance to fine compact grey limestone that the eye can probably not distinguish between them.

Forty-two grains of it lost 11.2 grs. in rain water at the