and Richter, have promised to continue their researches on the subject, some great and important fact must, he thinks, issue from their labours.

An Investigation of all the Changes of the variable Star in Sobieski's Shield, from five Years' Observations, exhibiting its proportional illuminated Parts, and its Irregularities of Rotation; with Conjectures respecting unenlightened heavenly Bodies. By Edward Pigott, Esq. In a Letter to the Right Hon. Sir Joseph Banks, K.B. P.R.S. Read February 7, 1805. [Phil. Trans. 1805, p. 131.]

Mr. Pigott, some years ago, presented to the Royal Society a paper, which is printed in the Philosophical Transactions for the year 1797, on the periodical changes of brightness of two fixed stars. The first part of the present paper consists of a series of observations made since those of the former paper, during the space of nearly five years, on one of the said stars, namely, that in Sobieski's Shield. These observations are fully detailed in various tables; and mean results are deduced from the observations given in the former paper, and from those described in the present one. The results are as follows:-Rotation of the star on its axis, 62 days.-Duration of brightness at its maximum, without any perceptible change, 9 days. -Duration of brightness when it does not attain its usual brightness, 20 days. Duration of brightness at its minimum, without any perceptible change, 9 days.-Ditto when it does not decrease so much as usual, 20 days.-Decrease in time, from the middle of its full brightness to the middle of its least, 33 days.-Increase of time, from the middle of its least brightness to the middle of its full, 29 days.-Extremes of its different degrees of brightness, 5th to 9th magnitude. Mean of its usual variation, 5th to 6th magnitude.

In the second part of this paper Mr. Pigott proceeds to examine some of the other phenomena belonging to this star, particularly one which, he says, is common to most of the variables, and likewise in some degree to our sun, namely, that the times of their periodical returns of brightness are, in general, irregular. In hopes of making some discovery respecting the cause of these irregularities, or at least of assisting future astronomers to form some opinion respecting them, Mr. Pigott made a series of observations on the star here treated of, beginning in October 1795, and ending in October 1801. These observations are detailed at full length in two tables; and it appears from them, that the periodical returns of brightness are uncommonly fluctuating, and that the differences between the extremes are very considerable. Mr. Pigott then, by way of explanation, offers the following opinions and inferences.

1st. That the bodies of the stars are dark and solid.

2ndly. That their real rotation on their axes is regular, following uniform impulses.

3rdly. That the surrounding medium does, at times, generate and

absorb its luminous particles, in a manner nearly similar to that which Dr. Herschel has supposed to take place with regard to the sun's atmosphere.

4thly. That as the variable star in Sobieski's Shield is occasionally diminished in appearance to the 6th or 7th magnitude, or even to a smaller magnitude, it appears that these luminous particles are but sparingly dispersed in its atmosphere.

5thly. He asks, may we not with much plausibility represent such luminous particles as spots, somewhat circular, and of no great extent.

6thly. That the principal bright parts are but slight patches, may, he says, be presumed, from the perpetual changes they undergo, and also from such changes being very visible to us.

7thly. He thinks we may obtain some idea of the relative situation or intervals between these bright parts, by the observations of the increase and decrease of brightness, as thereby the changes and times elapsed are pointed out.

Mr. Pigott says he has tried, practically, the effect of the above suppositions, by placing small white spots on a dark sphere, which sphere being turned round, represented the various changes as nearly as could be expected. Of these changes several views are given, accompanied with some observations on variable stars in general; in the course of which the author supposes it probable that many stars have lost their light, and that there are many others which have never shown a glimpse of brightness. He even asks, whether we may not suppose the number of these unenlightened stars equal to that of those endowed with light? If so, he thinks that by being collected together in clusters, as in the Milky Way, they must intercept all more distant rays; and if free from any intervening lights, must appear as dark spaces in the heavens, similar to what has been observed in the southern hemisphere.

Mr. Pigott, at the conclusion of his paper, says he thinks there are strong reasons to believe that the sun's luminous appearance has been at times considerably diminished; also, that he has little hesitation in conceiving it may, at some future period, be reduced to small patches.

An Account of some analytical Experiments on a mineral Production from Devonshire, consisting principally of Alumine and Water. By Humphry Davy, Esq. F.R.S. Professor of Chemistry in the Royal Institution. Read February 28, 1805. [Phil. Trans. 1805, p. 155.]

The mineral, of which an account is here given by Mr. Davy, was found many years ago by Dr. Wavel, in a quarry near Barnstaple. It was then considered as a kind of zeolite; but Mr. Hatchett, who visited the place in the year 1796, describes it as filling some cavities and veins in a rock of soft argillaceous schistus; and from that circumstance concluded, that it most probably did not belong to the

above-mentioned genus. Dr. Babington, from its physical characters, and from some experiments made on its solution in acids, ascertained that it was a mineral substance not yet described, and that it contained a considerable portion of aluminous earth.

This mineral is generally found in small hemispherical groups of crystals, composed of filaments radiating from a common centre, and inserted on the surface of the schistus: sometimes, however, it forms small veins of irregularly disposed prisms. It is of a white colour, having sometimes a tinge of gray, or of green; and, when beginning to be decomposed, of yellow. Its lustre is silky; it is generally almost opake, but sometimes semi-transparent. It is fragile; but its small fragments are so hard, as to be capable of scratching agate. It has no smell when breathed upon; it has not any taste, nor does it adhere to the tongue till it has been strongly ignited. It does not become electrical, or phosphorescent, by heat or friction; nor does it decrepitate before the blowpipe, but loses its hardness, and becomes quite opake. Its specific gravity, Mr. Davy thinks, does not exceed 270, water being considered as 100.

The white and semi-transparent specimens of this substance are soluble in the mineral acids, and also in fixed alkaline lixivia, without effervescence; but when coloured or opake specimens are exposed to alkaline lixivia, a small part remains undissolved.

A small transparent piece, by being exposed to the greatest heat of a forge, had its crystalline texture destroyed, and was rendered opake, but was not fused. It now had lost more than one-fourth of its weight, and adhered strongly to the tongue; neither water nor alcohol had any effect on this mineral. When exposed, in a glass tube, to a heat of from 212° to 600°, it gave out an elastic vapour, which, when condensed, was a clear fluid, having a slightly empyreumatic smell, but not differing in taste from pure water.

The solution of this substance in sulphuric acid produced crystals in thin plates, which had the properties of sulphate of alumine, and from which, when re-dissolved and mixed with potash, octahedral crystals of alum were obtained.

The solid matter precipitated from the solution of this substance in muriatic acid, was not acted upon by carbonate of ammonia, consequently it did not contain glucine or yttria.

Several experiments were made on the opake and coloured varieties of this mineral, from which it appears that the substances which cause these varieties, are calcareous earth, manganese, and oxide of iron.

Mr. Davy then proceeded to the analysis of the mineral. For this purpose he made use of the whitest and most transparent pieces he could obtain. The particulars of this analysis we shall pass over; and shall merely state that, according to its general results, 100 parts of the mineral contain, of alumine 70, of lime 1·4, of fluid 26.2, the loss amounting to 2:4; which loss Mr. Davy is inclined to attribute to some fluid remaining in the stone after the process of distillation,

having found, from several experiments, that a red heat is not sufficient to expell all the matter capable of being volatilized.

Mr. Davy then made some experiments to determine whether any portion of fixed alkali existed in this mineral, but no indications of such alkali could be observed.

The fluid obtained by distilling several different specimens of this mineral was similar in its properties; the only test of the presence of acid matter in it was litmus paper; and in some instances the effect upon this paper was scarcely perceptible. Mr. Davy made several experiments to determine the nature of the above acid matter, but without success.

It is, however, he says, evident that it is not any one of the known mineral acids: he is also disposed to believe, that it is not an essential component part of the mineral, but that, as well as the oxide of manganese, the oxide of iron, and the lime, it is only an accidental ingredient. Hence the mineral, when in a state of purity, must, he thinks, be considered as a chemical combination of about 30 parts of water, and 70 of alumine.

The diaspore, which has been examined by M. Vauquelin, loses 16 or 17 parts in the 100 by ignition, and contains nearly 80 parts of alumine, and 3 of oxide of iron. It is supposed by M. Vauquelin to be a compound of alumine and water. But its characters are very different from those of the mineral here described; and the nature of the part volatilized by heat has not yet been ascertained.

A mineral similar to that here treated of has been found near St. Austle in Cornwall; and Mr. Davy has been informed that, according to an analysis of it made by the Rev. William Gregor, it appears to consist of similar ingredients.

Dr. Babington has proposed to call this mineral by the name of Wavellite, from the gentleman who discovered it in Devonshire; but if a name founded upon its chemical composition should be preferred, Mr. Davy thinks it may be denominated Hydrargillite.

Experiments on Wootz. By Mr. David Mushet. the Right Hon. Sir Joseph Banks, K.B. P.R.S. 14, 1805. [Phil. Trans. 1805, p. 163.]

Communicated by
Read February

The fine cakes of the kind of steel called Wootz, which form the subject of the present paper, were delivered to Mr. Mushet, for the purpose of examination, by Sir Joseph Banks. Mr. Mushet begins his account of them by giving a very minute description of the form, the grain, and every other external character of these cakes. This description cannot well be abridged, and is too long to be repeated. We shall therefore only say that Mr. Mushet states, as a general remark, that the grain and density of these cakes of wootz were uniformly homogeneous, and free from metallic iron towards the under or round surface, but that they were always the reverse towards the upper side, called by Mr. Mushet the feeder.

The appearances observed upon forging these cakes are then par

ticularly described, from which Mr. Mushet deduces the following general remarks.

The formation of wootz, he says, appears to him to be in consequence of the fusion of a particular ore, which he supposes to be calcareous, or to be rendered so by a mixture of calcareous earth, along with a portion of carbonaceous matter. The fusion, he thinks, is performed in a clay vessel or crucible, in which vessel the separated metal is allowed to cool. Hence, in his opinion, arises the crystallization that occupies the pits and cells observed in and upon the under or round surface of the cakes.

The want of homogeneity and solidity in these cakes, appears to Mr. Mushet to be owing to the want of a sufficient degree of heat to effect a perfect reduction; and this opinion, he thinks, is strengthened by observing, that those cakes which are the hardest, or which contain the largest portion of carbonaceous matter, and, of course, form the most fusible steel, are always the most solid and homogeneous; while, on the contrary, those cakes which are the most easily cut by the chisel, are in general cellular, and abound with veins of malleable iron. If the natives of the country which produces the wootz were capable of rendering it perfectly fluid, Mr. Mushet thinks they would certainly have run it into moulds, by which, he says, they would have acquired a kind of steel more uniform in its quality, and more fit for the purpose of being worked and applied to the

arts.

Some of the cakes here described had, around the feeder, and upon the upper surface in general, evident marks of the hammer. This appearance Mr. Mushet accounts for by supposing, that when the cake was taken from the pot or crucible, the feeder was most probably slightly elevated, and the top of the cake covered in part with small masses of ore, which, from want of a sufficient degree of heat, had not been perfectly fused. These, he thinks, are cut off at a second heating, and the surface then hammered smooth, to make the cakes more fit for sale. Mr. Mushet says he has observed similar appearances in operations of a like nature, where the heat has been insufficient; and that such phenomena sometimes take place in separating crude iron from its ores, when, from its containing an excess of carbon, it is difficult to be fused.

The division of the cakes, by the native manufacturer, he thinks, is done merely to facilitate its subsequent application to the purposes of the artist, and to serve as a test of the quality of the steel.

In order to determine by direct experiment whether wootz owes its hardness to an excess of carbon, Mr. Mushet made some comparative experiments upon the cakes, and upon common cast steel and white cast iron. In operations of this kind, he says, he has always found the proportion of carbon best ascertained by the quantity of lead reduced from flint glass. He therefore mixed a certain quantity of wootz, or of steel, or iron, with three times the weight of pounded flint glass, and exposed the mixture to a heat of 160° of Wedgwood's pyrometer.