and the nature of its component parts, it is both heavy and tough, requiring a smart blow of the hammer to break it. Before the blowpipe, it fuses without much difficulty, into a pale-brown glass; it is not magnetic like basalt. At St. Anthon's colliery (see Geological Transactions, vol. iv. p. 41), the bed lies nearly 104 fathoms from the surface of the earth, and is a foot in thickness; its situation is between a stratum of strong white post (sandstone) seven feet thick, and a stratum of blue metal (shale, slate clay) eight and a half feet thick. In Walbottle Dene, five miles west of Newcastle, a similar stratum creeps ont by the edge of the brook; but its thickness or relative pusition cannot there beso accurately ascertained as at St. Anthony's, where a shaft has been sunk through it. Whether this be a continuation of the same bed, I cannot determine; but the high main coal cropping out at Benwell Hills, two miles to the east of Walbottle, and this bed lying 29 fathoms below that seam, I am inclined to consider them one and the same, notwithstanding the stone is finer grained at the latter place; for all our coal strata rise to the clay in succession towards the west and south-west.

It is by no means improbable that similar beds may be known to exist in other districts; but never having heard of a stratum of compact felspar and mica, as a member of a coal formation, you will oblige me by giving publicity to this short notice through the medium of your journal.

I have the honour to be, Sir,
Your most obedient servant,

N.J, Winch.

ARTICLE VI.

where VANDE

milor

Queries on the Plumbago formed in Coal Gas Retorts.

By the Rev. J. J. Conybeare, MGS.
(To the Editor of the Annals of Philosophy.)
NIY DEAR SIR,

Bath Easton, Dec. 15, 1822. The very general use of coal gas, and the degree of scientific information mostly to be found in those connected with its manu-. facture, render it probable that for many persons the remarks, which I am about to offer will possess but little of novelty. As, however, I am not aware that this subject has yet been noticed i in any periodical or oiher publication, I venture to intrude them on the notice of your readers, rather indeed in the hope of obtaining further information from those who are more competent than myself, than of adding much to the public stock,

an average

Rev.J. Conybeare on Plumbago in Coal Gas Retorts. 51 The cast-iron retorts used in the Bath gas works, being, on

somewhat less than one inch and a quarter in thickness, are burned through, as it is technically termed, in the part exposed to the greatest heat, in about 18 months. After this they are of course no longer serviceable. This destruction is effected by the progressive oxidation and scaling off of the metal. The scales consist chiefly of peroxide of iron, and the powder which they afford by grinding and washing might be applied to any of the purposes for which the crocus martis is in request. It is observable that the portion of cast-iron which reniains unoxidated in the parts exposed to a strong heat is found to present, on being broken, a texture much more highly crystalline than that of the portion further removed from the action of fire. * Is this simple development, or is it to be ascribed to that mysterious re-arrangement of particles which in some other cases appears to take place at a heat short of actual fusion?

1. The unserviceable retorts on being withdrawn from their beds are found lined with a coating of plumbago averaging the thickness of four inches. This coating is thickest towards the bottom of the retort, nearer to its mouth it becomes scantier and more intermixed with coaly and fuliginous matter. The general aspect of the predominant variety may be thus described : Colour, iron-grey, somewhat lighter than that of native plumbago ; texture, scaly; structure, mammillated, usually in very close aggregation ; some specimens exhibit this structure on the larger scale; generally it is discoverable only by the use of the lens; hardness, somewhat variable, but always greater than that of the best native plumbago, scratches gypsum, but is scratched by calc-spar; lustre of the exterior surface (especially where the mammillated structure is distinct), sometimes very considerable : lustre of the fracture usually but small: the powder uniformly resembles that of common plumbago, excepting that it is somewhat less brilliant.

In another variety, the structure of the mass is stalactitic, and its texture, where broken, perfectly compact and uniform; even under the lens, this variety closely resembles some of the stalactitic grey ores of manganese, and occasionally exhibits on the surface a considerable degree of iridescence. The mass for the greater part gives evidence of its gradual formation, by. its slaty. aspect, and tendency to break with greater readiness in the direction of what may be termed its strata. In many parts, it is rifty, and the rifts are usually coated with a brilliant deposition of a lighter colour than the general mass. Fragments projected

52 Rev.J. Conybeáre on Plumbago in Coal Gas Retorts, (JANI on nitre deflagrate, but not rapidly, leaving after combustion traces of iron. Some rude experiments gave me reason to think; that the quantity of iron varied in different specimens, and that it scarcely amounted, at the most, to the nine per cent. stated by Berthollet to exist in native graphite. That the substance in question is a true artificial plunibago admits, I apprehend, of no doubt. It must be formed plentifully in many other gas works besides those of Bath, and must, therefore, in all probability have attracted the notice of persons far better able to inquire into its production and properties than myself. In our works it has not yet been converted to any useful purpose. In writing or drawing, its hardness and general want of compactness render it in its crude state altogether unserviceable. When finely powdered and washed, it might be used in composition pencils. For the domestic uses to which plumbago is applied in coating grates, &c. I have found it to answer sufficiently well; but it is objected by the savantes in housewifery, that it has not a sufficient lustre, and would, therefore, I suppose, be seldom used by them; but where preservation only, and not ornament, was the object,* I have been informed that it has been applied with success to the purpose of covering razor strops. For that of diminishing friction, and for the manufactory of crucibles, furnaces, &c. it would, I should apprehend, answer sufficiently well

, for the latter especially. It appears from a paragraph in the article Coinage (Supplement to Encyclopædia Britannica), that the blue pots used in the mint are all of foreign manufactory, those made in this country containing too small an admixture of black lead. Is this to be attributed to the scarceness of that material, or to the want of sufficient tenacity in the English clay with which it is worked up, rendering its addition beyond a certain point impracticable ?

But whether this substance prove useful or worthless in an ecos nomical view, its occurrence cannot fail to strike the more speculative inquirer as adding one more to the many instances in which the unintentional products of art have been found to resemble those of nature, and as contributing, remotely at least, to throw fresh light upon one of the most controverted points of geological theory. Plumbago is well known to be among the most infusible of mineral substances. Now in the present cases, if not the whole of its mass, at least all those portions in which the mammillated structure is discernible, and yet more its stalactitic form, must have been brought to a state of fusion by a heat inferior to that at which cast-iron begins to run. Will this be

power, a Hundred

the better accounted for by the long duration of the heat, or by
assuming that this compound, like some others, is more fusible
at the moment when its constituents first enter into chemical
union. Should it be apprehended that no actual fusion whatso-
ever has taken place, the formation and consolidation of the sub-
stance by heat without fusion will still furnish the vulcanist
with a new point of analogy.
Believe me, my dear Sir, very truly yours,

J. Í. CONYBEARE.

P.S. The character of some portions of this plumbago has struck me as not unlike that ascribed to the points of charcoal altered and fused by voltaic electricity, in some late American experiments.

ARTICLE VII.

On the Compressibility of Water. By Prof. Oersted.*

Prof. OERSTED, several years since, laid before the Royal Society of Copenhagen some experiments on the compression of water, and showed at that time that this might be effected by a much smaller power than is generally supposed, provided the instrument was constructed according to the well-known principle, that a pressure acting upon a small surface of an enclosed liquid had the same effect as a power equally great, acting upon each similar part of the whole surface. For the compression of water he made use of a large cylinder of brass upon which one smaller was screwed, furnished with a well-fitted piston. He was, therefore, able to show the compression of water by a sınall power, quite as well as Abich and Zimmermann had done by many hundred pounds weight. To measure the power, a tube full of air, which was confined by mercury, was used, by which contrivance, of course, the air underwent the same pressure as the water from which it was separated by the mercury. According to the principle, that the compression of the air is in proportion to the pressing power, it was easy to calculate this power. But notwithstanding the great strength of tlie brass cylinder in which the water was compressed, it was possible that it might have given way, so that not only the compression of the water might have been measured, but a result obtained, in which the flexibility of the instrument

a

4

0000

was included. In the first experiment of M. Oersted, as well as in all those of former philosophers, Canton only excepted, any change of temperature, which might have happened during the experiment, had not been taken into consideration, which, however, in several respects was necessary, as it might even be supposed that heat was produced by the very compression. The excellent experiments of Canton, which, in later times, have been almost forgotten, were made with the pressure of condensed and rarified air. But every condensation or expansion of air is accompanied with an adequate elevation or diminution of temperature; it was, therefore, to be feared, that this ingenious philosopher had been deceived by this influence. He found the compression of water, at a pressure, equal to that of our atmosphere, to be between Tooboo and Tootoog of the bulk of the

1000000 water. The experiments of Canton possessed a great advantage over all which have recently been made, viz. that the vessel which contains the liquid to be compressed undergoes the same pressure both internally and externally; so that neither its form nor its size can be altered. Within a few years, Mr. Perkins, the ingenious inventor of the siderographia, has made some experiments, which have the same advantage as those of Canton; he included the tube of metal in which the water was to be compressed, in water which was exposed to the same pressure. His ingeniously contrived experiments will always be of considerable importance, because he has made them with a power, which philosopher seldom has at his disposal, viz. a pressure several

, hundred times exceeding that of our atmosphere. It was not, however, his intention to ascertain by these experiments, whether heat was produced by the compression of water, and what influence it had upon the result. Prof. Oersted endeavoured, therefore, to contrive an instrument which allowed an exact measurement of the compressing power, as well as of the compression of the water itself, and

which at the same time made it easy, exactly to ascertain the influence which heat might have on the effect. The water which is to be compressed is included in a glass tube, d, which holds about two ounces of water: it is closed below, and its upper part terminates in a capillary tube, bc, 52 French lines long, and of even bore, so that the vessel is like à flask, the neck of which is a long capillary tube. On the upper

end of this tube is a small funnel two lines wide. The flask holds 709.48 grammes of mercury, but the mercury which fills 24:6 lines of the capillary tube weighs only 96 milligrammes, which gives Toolboot for the length of a line, or, to be more exact, 0.000005501 of the contents of the flask. When the experiment is to be made, the flask is warmed a little by being kept for a moment in the hand; if possible the temperature must not rise above 4 centigrade. Then a drop of mercury is introduced into the funnel, which, while the water in the flask is