they have been encouraged, there are few commons, no hedges, no shepherds, no dogs to attend the flocks; and the fences of wood and stone are not sufficient to prevent active sheep from breaking pasture, to the great destruction of adjacent crops.

The advantage, however, in this respect, is counterbalanced by a corresponding inconvenience arising from the same source, namely, the great difficulty of driving such cripples to market, at the same time that they are generally not so fat as others, from the greater labour they undergo in gathering their food.

These objections are indeed such, that since the introduction of Merinoes, which are equally gregarious, quiet, and orderly, in addition to the strong recommendation of their fleeces, the ancon breed appears in danger of becoming wholly extinct; so that the author had some difficulty in procuring one in Boston to be dissected, for the purpose of sending a skeleton, which accompanied the letter, and was laid before the Society.

Experiments to ascertain the coagulating Power of the Secretion of the gastric Glands. By Sir Everard Home, Bart. F.R.S. Communicated by the Society for promoting the Knowledge of Animal Chemistry. Read January 21, 1813. [Phil. Trans. 1813, p. 96.]

It has long since been observed, that the first step in the process of digestion is the conversion of the food into a jelly; but whether this is effected by means of the gastric liquor alone, or by a joint operation of other secretions, has not been ascertained. From Mr. Hunter's experiments, it appeared that the same species of coagulation takes place in the same food admitted into the stomach of a great variety of animals; and that in the calf's stomach this power resided in the fourth cavity alone; since the mucus taken from the surfaces of the first, second, or third cavities, had no such effect as rennet, which is prepared by infusion of the inner membrane of the fourth cavity.

The same inquiry is here pursued by the author, with a view to ascertain more accurately what part it is that possesses this property in the highest degree, by comparison of the effects of rennet prepared from different portions.

By this mode of trial no part of the hog's stomach was found to coagulate milk, but that near the pylorus, where the gastric glands are situated.

Experiments were next made with rennets prepared from the crop and gizzard of a cock, from the stomach of a shark, the stomach of a salmon, and that of a thornback, all of which had the power of coagulating milk.

Other experiments were afterwards made, with the assistance of Mr. Hatchett and Mr. Brande, on the comparative powers of different parts of the same stomach, and the difference in various species of animals, the chicken, hawk, turkey, and calf.

In a chicken the horny lining of the gizzard gave a firmer curd

from milk than the cardiac portion of the stomach. The cardiac portion of the stomach of the hawk was found more powerful than the same part of a common fowl.

The gastric glands were carefully dissected out from behind the membrane that lines the cardiac extremity of the stomach of a turkey; and of these, forty grains, by weight, were taken, and their effect compared with an equal weight of membranous lining of the same cavity, an equal weight of membrane from the fourth cavity of a calf's stomach in a recent state, and forty grains of dry rennet. Since the last must have been prepared from about four times its weight of recent membrane, its effect was produced in much the shortest time. The coagulation effected by the gastric glands took place nearly at the same time as by the recent calf's stomach; while that from the lining of the turkey's stomach was nearly three times as long in producing the corresponding effects.

From these experiments, the author infers that the secretion from the gastric glands possesses the power of coagulating milk, and communicates that property to adjacent parts, by which it is imbibed.

On some Properties of Light. By David Brewster, LL.D.F.R.S.Edin. In a Letter to Sir Humphry Davy, LL.D. F.R.S. Read January 28, 1813. [Phil. Trans. 1813, p. 101.]

The author, having been for some time past engaged in a course of experiments on the refractive and dispersive powers of different substances, the details of which are intended for future publication in a separate work, confines himself, at present, to a relation of such of his results as have most of novelty or importance. After repeating the experiments that have been made by others on the properties that light acquires by transmission through Iceland-spar, and upon the corresponding properties of reflected light originally discovered by Malus, and by him termed polarization, Dr. Brewster observed a singular appearance of colour on each side of a luminous object, viewed through a thin slice of laminated agate. Upon examination of these coloured images through a prism of Iceland-spar, this light was found to be similarly polarized, so as to appear or disappear accordingly as the lamina of the agate were parallel or transverse to the principal section of the spar. He found also that the colourless light transmitted directly through the agate, and from which the coloured rays had been separated, was polarized as well as the coloured rays, appearing and disappearing alternately with them during the revolution of the spar. And accordingly when light previously polarized by reflection was received upon the agate, its transmission or reflection depended on the relative position of the lamina of the agate to the plane of reflection; for when these were at right angles to each other, no light whatever was transmitted.

In the same manner light polarized by transmission through the laminated agate, manifested the usual properties of light so affected by other means.

Along with the polarized light, Dr. Brewster also observed a faint nebulous light not polarized, which he also finds in transmission through cornelian and chalcedony, and thinks it important as leading to a satisfactory theory of polarization.

The next observation of the author relates to the high refractive power of chromate of lead, which, he remarks, is greater than that of any other body hitherto recorded; and upon its double refraction, which, he says, is so enormous, that the deviation of the extraordinary ray is more than thrice that produced by Iceland-spar.

The index of refraction assigned by Dr. Brewster to chromate of lead, is 2.926, and along with it he names realgar, of which the index is 2.510, as another substance that refracts more strongly than the diamond. Phosphorus, he adds, stands higher than has been supposed, being 2-224, and then native sulphur next in order 2.115.

The dispersive power of chromate of lead is observed to exceed that of other substances in a still greater proportion than its refractive power, being more than ten times as great as that of the densest flint-glass, and fifteen times as high as that of water.

The concluding section of the author's letter relates to the existence of two dispersive powers in all doubly refracting media. Mr. Cavallo, and others, have already observed, that the dispersions occasioned by the two refractions of Iceland-spar are not equal. Dr. Brewster observes that this is general to all, and he undertakes to assign the proportion of these two powers in different substances.

In chromate of lead the dispersive power manifested in the rays ordinarily refracted, is double that of rays obliquely refracted; and in Iceland-spar the disparity is nearly as great.

The existence of a double dispersive power, it is observed by the author, instead of assisting in the explanation of other properties, only adds one to the numerous difficulties that are to be surmounted in reducing to any general rules those capricious phenomena exhibited by light in its passage through transparent bodies.

An Appendix to Mr. Ware's Paper on Vision. By Sir Charles Blagden, F.R.S. Read February 4, 1813. [Phil. Trans. 1813,

p. 110.] The author remarks, that Mr. Ware's observations with regard to short-sightedness, being in general merely the consequence of habit acquired at an early age, is conformable with his own experience in general, and that he himself is a particular instance of natural longsightedness gradually converted into confirmed short sight. He very well remembers first learning to read, at the common age of four or five years, and that at that time he could see the usual inscriptions across a wide church; but that at the age of nine or ten years he could no longer distinguish the same letters at the same distance, without the assistance of a watch-glass, which has the effect of one slightly concave. In a few years more the same glass was not sufficiently powerful; but yet his degree of short-sightedness was so inconsiderable, that he yielded to the dissuasion of his friends from

using the common concave glasses till he was upwards of thirty years of age, when No. 2 was barely sufficient; and he very shortly had recourse to No. 3. In the course of a few years an increase of the defect rendered it necessary for him to employ glasses still deeper, and his sight soon required No. 5, where it has remained stationary to the present time. From the progress which Sir Charles Blagden has observed in his own short-sightedness, he is of opinion that it would have been accelerated by an earlier use of concave glasses, and might have been retarded, or perhaps prevented altogether, by attention to read and write with his book or paper as far distant as might be from his eyes.

In this communication he takes the same opportunity of adding an experiment made many years since on the subject of vision, with a view to decide how far the similarity of the images received by the two eyes contribute to the impression made on the mind, that they arise from only one object. In the house where he then resided, was a marble surface ornamented with fluting, in alternate ridges and concavities. When his eyes were directed to these, at the distance of nine inches, they could be seen with perfect distinctness. When the optic axes were directed to a point at some distance behind, the ridges seen by one eye became confounded with the impression of concavities made upon the other, and occasioned the uneasy sensation usual in squinting. But when the eyes were directed to a point still more distant, the impression of one ridge on the right eye corresponded with that made with an adjacent ridge upon the left eye, so that the fluting then appeared distinct and single as at first, but the object appeared at double its real distance, and apparently magnified in that proportion. Though the different parts of the fluting were of the same form, their colours were not exactly alike, and this occasioned some degree of confusion when attention was paid to this degree of dissimilarity.

A Method of drawing extremely fine Wires. By William Hyde Wollaston, M.D. Sec. R.S. Read February 18, 1813. [Phil. Trans. 1813, p. 114.]

The author refers to Musschenbroek for an instance of a gold wire, recorded to have been drawn by an artist at Augsburg so fine, that one grain of it would have the length of 500 feet. It is not said how this was effected, and some doubt has been entertained of the possibility of it; but the author of this paper shows how gold may be drawn to the same degree of fineness, and also that platina may be made with great facility much finer than is above described.

The general principle of the method is the same for both. The metal intended to be drawn is first reduced, in the common mode, to a wire of about th of an inch in diameter; and it is then coated with silver, so as to form a rod of considerable thickness. The rod is then drawn, as usual, till it is reduced to a slender wire, and it is presumed that the gold or platina contained in it is reduced in the

same proportion as the silver. By steeping for a few minutes in nitrous acid the silver is then dissolved; but the gold or platina remain unaffected, and require merely to be washed in distilled water in order to free them from any portion of the solution or other little impurities that may adhere during the solution.

The method employed by the author for coating gold wire is attended with more difficulty than he expected. A rod of silver having been previously drawn of considerable thickness, a hole was drilled through it longitudinally, and into this hole a gold wire was inserted so as to fill the hole. But in consequence of the toughness of fine silver, the operation of drilling was found extremely difficult, and this method was afterwards abandoned. It was found that platina might be advantageously substituted for gold, as in that case the first drawn wire might be coated with silver by fixing it in the axis of a cylindrical mould, and then pouring melted silver to fill the mould. The platina employed for this purpose was fused by the flame of a spirit lamp impelled by a current of oxygen, as contrived by Dr. Marcet: this platina having then been drawn alone to a wire

of an inch in diameter, it received a coating of silver that was just 80 times the thickness of the platina: accordingly when the silver was reduced by drawing to of an inch in diameter, that of the platina was ; but nevertheless it remained surprisingly tenacious in proportion to its substance. The greatest relative tenacity is however thought to have been at about T of an inch, which supported 1 grain before it broke. Accordingly this wire admitted being drawn considerably finer, and the author has even obtained portions as slender as of an inch; but these were only in very short pieces, being in many places interrupted so that he could place no reliance upon any trials of their tenacity.

Some precautions are added respecting the method of freeing these wires from their coating of silver, with the recommendation of some little contrivances which the author has found convenient in handling objects so liable to be injured.

Description of a single-lens Micrometer. By William Hyde Wollaston, M.D. Sec. R.S. Read February 25, 1813. [Phil. Trans. 1813, p. 119.]

The author, being unable to measure some of his very small wires so accurately as he wished by any means at present in use, contrived the method here described, which he recommends as fully answering his expectations.

A lens having its focus at one twelfth of an inch is mounted in a plate of brass, and by the side of it is made a small perforation, as near to its centre as th of an inch.

When a lens thus mounted is placed before the eye for the purpose of examining any small object, the eye can at the same time see distant objects through the adjacent perforation, by reason of the magnitude of the pupil, which is sufficient for receiving rays through