property belongs, I instituted the following experiments, which were made by my fellow labourers in animal chemistry, Mr. HATCHETT and Mr. W. BRANDE.

On the 9th of June, 1812, Mr. HATCHETT took the cardiac portion of a chicken's stomach with the gastric glands that open into it, and put it into a glass vessel; the horny lining of the gizzard was put into another, milk was added to each, and was converted into a curd; but the curd in the vessel containing the lining of the gizzard was the firmest.

Mr. BRANDE, on the 12th of June, 1812, made a similar experiment with the cardiac portion of the stomach of the hawk, as a carnivorous bird, in one vessel; and the cardiac portion of the fowl, as a granivorous one, in another. The coagulating power of the hawk's stomach was found to be the most powerful.

To ascertain whether the coagulating power belongs to the secretion of the gastric glands, and is only communicated to the other parts, I instituted the following experiment, which was made by Mr. BRANDE on the 13th of July, 1812. I selected the turkey for the subject of it, as the gastric glands of that bird are larger than in most others. The turkey had been kept one day without food before it was killed, and immediately after death the gastric glands were very carefully dissected from each other, on the outside of the membrane which lines the cardiac cavity without opening into it, each gland was then separately removed by cutting through the excretory duct, leaving the cardiac cavity entire and unopened.

Of these glands slit open 40 grains were put into a vessel, and two ounces of new milk added.

40 grains of rennet were put into a second vessel, to which the same quantity of milk was added.

40 grains of the lining of the cardiac cavity of the turkey, and the same quantity of milk were put into a third vessel.

40 grains of the fourth cavity of a calf's stomach in a recent state, with the same quantity of milk, were put into a fourth vessel.

The experiment commenced at ten o'clock in the morning, at half past ten the milk with the rennet became thick, at twelve curd was formed, at two whey separated, at four the formation of curds and whey appeared complete.

At half past eleven the milk with the glands became thick, at one curd was formed, at three whey separated, at six the formation of curds and whey was complete.

The milk with the portion of the recent calf's stomach underwent the same changes at the same periods.

The milk with the cardiac membrane of the turkey at four became thick, at eight curd was formed, the separation into curds and whey was not complete till next morning.

A portion of the same milk in twenty-four hours had undergone no change, except that cream had separated.

The rennet in a dried state consisted of four times the quantity of membrane employed in the experiment with the recent calf's stomach, which accounts for its more readily producing coagulation.

From these experiments, it is clear that the secretion of the gastric glands possesses the power of coagulating milk, and gives that power to all the parts by which it is imbibed, whether composed of living parts or not, since the horny lining O 2

of the gizzard, the mucus in the stomach, and the inner membrane of that cavity appear equally to have acquired it.

This coagulation appears to be the first change the food undergoes in the process of digestion, and where the digestion is rapid, the coagulated parts are very quickly dissolved.

Mr. BULLOCK was led, by his love of natural history, to spend some time on the Bass Rocks, and has frequently seen a Solan goose swallow a herring, and come immediately to feed its young, and although the time the herring remained in its stomach could not have been more than a few minutes, when it was brought up again, to be given to the young bird, the external covering was entirely dissolved.

XIII. On some Properties of Light. By David Brewster, LL.D. F.R. S. Edin. In a Letter to Sir H. Davy, LL.D. F. R. S.

DEAR SIR,

Read January 28, 1813.

HAVING been for some time engaged in a series of experiments on the phenomena of light arising from its transmission through diaphanous bodies, I have taken the liberty of communicating to you, for the information of the Royal Society, a short and general account of the results of my enquiries. In the narrow compass of a letter, it would be impracticable to include the various details of these experiments; the particular methods of observation that were employed; or the numerical results which I have obtained for the refractive and dispersive powers of nearly two hundred substances. As these will form part of a separate work, in which I am now engaged, I shall confine myself at present to some of those results which appear to be most interesting, either from their novelty or importance.

1. On a new Property of refracted Light.

As you are already well acquainted with the optical properties of doubly refracting media, and the analogous property of reflected light discovered by MALUS, it will be unnecessary to take any notice of these phenomena. After repeating the experiments of MALUS, and measuring several of the angles

of incidence at which this property was communicated to light by reflection from different substances, I made a variety of experiments, with the view of discovering if a similar character could be impressed upon light by its transmission through bodies, either wholly or imperfectly transparent. All these experiments afforded no new result, and every hope of discovering such a property was extinguished, when my attention was directed to a singular appearance of colour in a thin plate of agate. This plate, bounded by parallel faces, is about the fifteenth of an inch thick, and is cut in a plane perpendicular to the lamina of which it is composed. The agate is very transparent, and gives a distinct image of any luminous object; but on each side of this image is one highly coloured, forming with it an angle of several degrees, and so deeply affected with colour that no prism of agate, with the largest refracting angle, could produce an equivalent dispersion. Upon examining this coloured image with a prism of Iceland spar, I was astonished to find that it had acquired the same property as if it had been transmitted through a doubly refracting crystal, and upon turning the Iceland spar about its axis, the images alternately vanished at every quarter of a revolution. My attention was now directed to the common colourless image formed by pencils transmitted perpendicularly through the agate; and by viewing it through a prism of Iceland spar, it exhibited all the characters of one of the pencils produced by double refraction, the images alternately vanishing in every quadrant of their circular motion.

When the image of a taper reflected from water at an angle of 52° 45′, so as to acquire the property discovered by MALUS, is viewed through the plate of agate, so as to have its laminæ