served to indieate 27.76. The mercurial column had sunk two inches; for in the register kept at the Start Point Light-house, about a mile from Lopness, the marking at 8 A. M. was 29.68. When the cloud had completely passed, Mr Lindsay, studentof Divinity, having gone into the garden at Lopness, observed that the "cabbages were perforated as if musket-bullets had been shot against them."" About an hour afterwards," he adds, "I picked up some that still remained undissolved, and found that they measured 1th inch in diameter. They were for the most part of a spheroidal form, consisting of a nucleus resembling common hail, occupying about }d of the diameter, encrusted by a coating of transparent ice. Some of the stones, however, were irregularly formed into a sort of crystallized mass."

ART. XXVI.-Abstract of Mr HERSCHEL'S Experiments on Circular Polarisation *.

IN the 3d volume of this Journal, p. 897. we have already given a brief abstract of the important results contained in Mr Herschel's paper on circular polarisation; and our readers will find in Vol. II. p. 179, 180. a short notice of what had been pre-viously done on the same subject t.

Mr Herschel's mem ir is limited to an examination of the properties of that variety of quartz to which Haüy has given the name of plagiedre. One of these crystals is represented in Plate VII. Fig. 7., where the faces x, x, x, and x', x', x', peculiar to this variety, lean, as it were, in one uniform direction round the summit A, which is adjacent to them, the angle formed by these faces, and the adjacent sides of the prism, being greater on one side (the right, for instance,) than it is on the other. If the summit a is placed uppermost, by inverting the crystal, the plagiedral faces adjacent to this summit obey the same law, and turn in the same direction.

This ingenious paper, read April 17. 1820, will immediately appear in the Transactions of the Cambridge Philosophical Society, vol. i.

See also p. 421. of this Number.

"The faces in question," says Mr Herschel," originate in those laws of decrement which Haüy has called intermediate. The primitive form of quartz is a rhomboid, slightly obtuse, whose axis is parallel to that of the hexagonal prism. The subtractive molecule by which the decrement on the angles E (Fig. 8.) takes place, to produce the faces x, is composed of eight of these rhomboids, its edges consisting respectively of one, two and four edges of the primitive rhomboid; and the decrement resulting is represented in Haüy's notation by (ED2D1). The alternate faces a arise from a different law, (as they obviously must, the angles upon which they are produced being differently related to the superior vertex). Their law of decrement cannot be reduced to an integer expression, but is represented by (E D'B'), in the same notation *."

Upon cutting plates perpendicular to the axis of this interesting variety of rock-crystal, Mr Herschel found that the direction of the polarisation was constantly the same as the direction of the plagiedral PLANES, the polarisation being direct or retrograde, according as these faces leaned forward or backward round the summit. The number of crystals which he examined amounted to no fewer than twenty-three, and in all these, without a single exception, the direction of the polarisation was the same as the direction of the faces," although M. Biot," as Mr Herschel remarks," has assured us that no peculiarity in the crystalline form can lead us to conjecture what may prove the direction of rotation in a given specimen of rock-crystal previous to trial,"

Notwithstanding the generality of the fact discovered by Mr Herschel, he observed in the possession of Mr Brooke a crystal of quartz, which exhibited on one and the same angle of the prism plagiedral faces perfectly distinct and in contact, but leaning opposite ways round the summit. We trust that Mr Brooke will sacrifice this specimen, or rather a part of it, to the good of science, and enable Mr Herschel either to establish or overturn the important relation which he has discovered. We venture to say, with confidence, that this crystal is a

See Haüy's Traité de Mineralogie, 4to, Plate 45. and tom. ii. p. 297,

colourless amethyst, of which we are in possession of several spe

cimens.

Another experiment of Mr Herschel is, in our opinion, equally hostile to M. Biot's notion, that circular polarisation is an inherent property of the ultimate particles of matter*. He prepared the Liquor Silicum, (a solution of silica in potash), from a portion of a plagiedral crystal, which turned the plane of polarisation to the left; but it possessed no circular polarisa

tion.

Arguments of a similar kind had been urged by Dr Brewster against the opinion of the French philosopher. When he discovered more than five years ago the double system of rings in crystallized sugar, he observed that there was no circular polarisation at the two poles, although solutions of sugar were known to possess that property; and in his paper on the Ame thyst, he states, that neither Opal nor Tabasheer (which is nothing more than the Liquor silicum solidified,) have the rotatory property of quartz. Another argument still more convincing will be found in our Scientific Intelligence, from which it pears, that Dr Brewster has examined a piece of Melted Quartz, which had been entirely deprived of its ordinary polarising structure, and has found that it exhibits no traces of circular polarisation. Each of these facts we regard as an experimentum crucis sufficient to decide the question; and we have brought them forward at present, because Mr Herschel seems to think it possible that the rotatory property may be inseparable from the ultimate particles of the body which exhibits it.

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• Faculté qu'elles ne peuvent perdre que lorsqu' elles cessent d'etre elles-memes, par leur decomposition.-Mem. Inst. 1818.

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ART. XXVII.-Observations on the Nature of Flame, drawn from several Experiments performed with an Apparatus for Discharging Ordnance without the use of a Match or PrimeTube. By JOHN DEUCHAR, M. W. S., and Lecturer on Chemistry in Edinburgh. Communicated by the Author *. THE apparatus with which the following experiments were performed, was suggested by Lieut.-Col. Udney Yule, for discharging ordnance upon Mr Forsyth's plan, and will be understood from Plate IX., and the explanation of it given at the end of this paper. In all these experiment, the new fulminating powder used, was exploded by a stroke from a wooden hammer weighing nearly 1 lb. The stroke was applied to that part marked C of Fig. I. and CD was brought down previous to the discharge, so as to rest upon I. The end F was rested upon a table, and the other end E was held in the hand.`

I. I first directed my attention to such experiments as I thought most satisfactory in proving the application of the apparatus to the firing of ordnance of every description; and for this purpose the first seven experiments have been selected.

Exp. 1.-A piece of flannel was put over the bottom of a tube 15 inches long (See Pl. IX. Fig. 1. B.), and immediately below, and close to it, was tied two folds of paper, with a quantity of gunpowder. Upon exploding a grain of the new fulminating powder at the top (A), the flame was forced down the whole tube, and the gunpowder was fired. When the gun

powder is wrapped in a single piece of thin paper, it often happens that the flame forces through without firing it. When this takes place, the whole or a part of the gunpowder is scattered about, and the paper is rent asunder, without any appearance of combustion.

Exp. 2.-The first experiment was repeated, the gunpowder being surrounded by flannel. Upon exploding the fulminating powder at the top, the flame pierced the flannel, and inflamed the gunpowder.

Both these experiments prove, that the flame of the new fulminating powder can descend through a tube 15 inches deep,

• This communication is drawn up from three papers on the subject read before the Wernerian Natural History Society, and published in their Memoirs, vol. iii.

pierce a piece of flannel, and fire gunpowder. And supposing the tube to represent the touch-hole of a gun, and the flannel and gunpowder to be a substitute for the cartridge, then we may conclude that the gun would be discharged, although the cartridge-were 15 inches distant from the fulminating powder, which never occurs, even in the largest pieces of ordnance. An objection, however, arises as to the above conclusion; that it may be owing to the tying being very close, and the flame having no room to spread, that the gunpowder was inflamed in the 1st and 2d experiments, and that when applied to the gun the fine may be lost over the surface of the cartridge. To answer this objection, the two following experiments were performed.

Exp. 3.-A quantity of gunpowder was scattered over the bottom of a circular tin canister, (See Pl. IX. Fig. 1. KLMN.) 8 inches deep, and nearly 3 in diameter. Over the powder was laid a piece of cartridge flannel. The tube (AB) was made to descend into the canister, to within 2 inches of the flannel; and then a grain of the fulminating powder was exploded at the top. The result was quite satisfactory, the flame pierced the loose flannel, and fired the gunpowder.

Exp. 4.-A tube 14 inches long, was bent at the 10th inch from the top, so as to present 4 inches out of the straight line in which the flame formerly proceeded, and through which it must now pass before it could issue from the under end, (See Plate IX. Fig. 1. OP:) Upon exploding the powder as before, the flame issued from the bent end of the tube.

Exp. 5.-In order to ascertain whether the apparatus were apt to clog up, or miss firing from repeated use, it was discharged 130 successive times, and never failed to produce the proper effect; and still the apparatus did not require to be cleaned for future use.

Exp. 6.-In order to determine the temperature at which the new fulminating powder would spontaneously explode, a number of experiments were made. In one of these there were placed upon a circular tim plate, at three situations, with 2 inches intervening, forming, as it were, the points of an equilateral triangle, sulphur, gunpowder, and the fulminating mixture, one grain of each; below the centre of this triangle, was put a ta