1. Chromate of lead. 2. Carbonate of lead. 3. Zircon. 4. Pistazite. 5. Carbonate of strontian. 6. Chrysolite. 7. Calcareous spar. 8. Topaz. 9. Tartaric acid. 13. Sulphate of iron. Some years ago Malus, a colonel of engineers in the French army, announced the discovery of a new property of reflected light. He found that when light is reflected at a particular angle from all transparent bodies, whether solid or fluid, it has acquired by reflection that remarkable property of polarisation, which had hitherto been regarded as the effect only of double refraction. If the light of a taper, reflected from the surface of water at an angle of 52° 45', be viewed through a rhomboid of Iceland crystal which can be turned about the axis of vision, two images of the taper will be distinctly visible at one position of the crystal. At the end of of a revolution one of the images will vanish, and it will re-appear at the end of of a revolution. The other image will vanish at the end of of a revolution, and will re-appear at the end of; and the same phenomena will be repeated in the other two quadrants of its circular motion. The light reflected from the water therefore has evidently been polarized, or has received the same character as if it had been transmitted through a doubly refracting crystal. The angle of incidence at which this modification is superinduced upon reflected light increases in general with the refractive power of the transparent body; and when the angle of incidence is greater or less than this particular angle, the light suffers only a partial modification, in the same manner as when two rhomboids of Iceland spar are not placed either in a similar or in a transverse position. Malus found that light reflected from opaque bodies, such as black marble, ebony, &c. was also polarized. But polished metals, according to him, did not impress that property, though they did not alter it when it had been acquired from another substance. Dr. Brewster, however, has observed, that polished metals polarize light as well as other substances. When a ray of light was divided into two pencils by a rhomboid of Iceland spar, Malus made these pencils fall on a surface of water at an angle of 52° 45'. When the principal section of the rhomboid (or the plane which bisects the obtuse angles) was parallel to the plane of reflection, the ordinary pencil was partly reflected, and partly refracted, like any other light; but the extraordinary ray penetrated the water entire, and not one of its particles escaped refraction. On the contrary, when the principal section of the crystal was perpendicular to the plane of reflection, the extraordinary ray was partly refracted and reflected, while the ordinary ray was refracted entire. While Dr. Brewster was employed in repeating the experiments of Malus, and observing the effect produced upon light by transmitting it through transparent and imperfectly transparent bodies, he was struck by a singular appearance of colour in a plate of agate. This plate, bounded by parallel faces, was about the 15th of an inch in thickness, and was cut in a plane perpendicular to the lamina of which it was composed. This agate was very transparent, and gave a distinct image of any luminous object. On each side of this image was one highly coloured, forming with it an angle of about 10°, and so deeply affected with the prismatic colours that no prism of agate, with the largest refracting angle, could produce an equivalent dispersion. Both the coloured images and the colourless image were found to be polarized. Dr. Brewster found that when the image of a taper, reflected from water at an angle of 52° 45′, is viewed through a plate of agate, having its laminæ parallel to the plane of reflection, it appears perfectly distinct; but when the agate is turned round, so that its laminæ are perpendicular to the plane of reflection, the light which forms the image of the taper suffers total reflection, and not one ray of it penetrates the agate. He found likewise that if a ray of light incident upon a plate of agate be received after transmission upon another plate of the same substance, having its laminæ parallel to those of the former, the light will find an easy passage through the second plate; but if the second plate has its laminæ perpendicular to those of the first, the light will be wholly reflected, and the luminous object will cease to be visible. But the most curious observation made by Dr. Brewster on the agate is the presence of a faint nebulous light, unconnected with the image, though always accompanying it, lying in a direction parallel to the lamina. This unformed light never vanishes along with the images; and in one of the specimens of agate it is distincely incurvated, having the same radius of curvature with the adjacent laminæ. Dr. Brewster found the same property in the carnelian and chalcedony, minerals of which the gate is usually composed. Dr. Brewster ingeniously conjectures that the structure of agate is an approach to that particular kind of crystallization which occasions double refraction, and that the nebulous light is an imperfect image arising from that imperfection of structure. He conceives that the phenomena of double refraction are produced by an alternation of laminae of two separate refractive and dispersive powers. Thus in calcareous VOL. I. N° IV. U spar, one set of laminæ may be composed of lime, the other of carbonic acid. The only double refracting crystal incompatible with this supposition is sulphur, which, however, may hereafter be ascertained to be a compound. Another very singular discovery of Dr. Brewster is, that when polarized light is transmitted through certain transparent bodies, it is unpolarized by these bodies in certain positions, and unaltered by them in others. The transparent bodies which possess this property are rock crystal, topaz, chrysolite, borax, sulphate of lead, felspar, selenite, citric acid, sulphate of potash, carbonate of lead, leucite, tourmaline, pistazite, mica, Iceland spar, agate without veins, some pieces of plate glass. Gum arabic, horn, glue, and tortoiseshell, depolarize light in every position. m A ? G Dr. Brewster has observed that miça and topaz exhibit some singular phenomena with light. Let the rectangle ABCD represent a plate of mica. When a prism of calcareous spar is placed in a vertical, or horizontal line, upon this plate, polarized light viewed through them both suffers no change. The horizontal and a E D H vertical lines EF, G H upon the plate of mica may be called the neutral axes of the mica. When the Iceland spar is placed in the diagonals AC, BD of the plate, the polarized light is depolarized, and hence these diagonals may be called depolarizing axes. If we examine a polarized image by the prism of Iceland spar, placed upon the vertical neutral axis of the mica, the polarity of the light will of course continue, and only one image will be seen; but if we incline the plate of mica forwards, so as to make the polarized light fall upon it at an angle of about 45°, the image that was formerly invisible starts into existence, and therefore the light from which it was formed has been depolarized. If the same experiment is made upon the horizontal neutral axis, no such effect is produced; and hence it follows that the vertical neutral axis is accompanied by an oblique depolarizing axis. By making the same trials with the depolarizing axes, it will be found that each is accompanied by an oblique neutral axis; and therefore each plate of mica possesses two oblique neutral axes, and one oblique depolarizing axis. The oblique depolarizing axis is represented by the line on, and the two oblique neutral axes by the lines om and op. The angles Gon, Gom, Gop, being about 45°, and the planes of these angles being perpendicular to the plate of mica. Topaz was found to exhibit the same phenomena to a limited extent; but no other substance tried. We have still some other discoveries, made by Dr. Brewster, on light, to mention; but this article has already extended to such a length that we must delay our account of them till our next number. III. Matches that take fire when dipped into sulphuric acid. In answer to our Weymouth correspondent, who requests us to inform him what is the composition of these matches, we answer, that such matches are by no means new; they have been known for many years to chemists. They are composed of the hyperoxymuriate of potash in powder, mixed with sugar or charcoal powder. The whole must be well mixed together. But whoever makes them must beware of rubbing them hard in a There is no occamortar, for such a mixture is apt to explode. sion for much nicety about the proportions. I have often made such matches, eight or ten years ago, and then employed equal weights of the two ingredients. IV. Population of Sunderland. A correspondent from Sunderland has obliged us with a correction of our statement of the population of that town in our last number. It consists, he says, of three parishes united, namely, Sunderland, Bishopwearmouth, and Monkwearmouth; and the population of all the three, which constitutes the town of Sunderland, exceeds, he says, 30,000. We have not the parliamentary returns for 1811 at hand; but have no doubt of the accuracy of our correspondent. V. Geognosy of Werner. In our last number we inserted an admirable paper by Professor Jameson, of Edinburgh, vindicating the geognosy of Werner from the attack made upon it by the Edinburgh Review, Professor Jameson maintained, against the opinion of the reviewer, that Haüy and Brogniard, in their account of the environs of Paris, had adopted the conclusions, and used the language, of the Wernerian geognosy. Had he seen the Recherches sur les Ossemens fossiles de Quadrupeds, published by Hauy in 1812, he would have found the following passage, which deserves to be quoted as a vindication, or rather demonstration, of Professor Jameson's opinion :-"En effet, la partie purement minerale du grand probleme de la theorie de la terre a eté etudiée avec un soin admirable par de Saussure, et portée depuis à un developement etonnant par M, Werner et par les nombreux et savans eleves qu'il a formés-Le second (Werner) profitant des nombreuses excavations faites dans le pays du monde ou sont les plus anciennes mines, a fixé les loix de succession des couches; il a montré leur ancienneté respective et pursuivi chacune d'elles dans toutes ses metamorphoses. C'est de lui, et de lui seulement, que datera la geologie positive, en ce qui concerne la nature minerale des couches; mais ni l'un ni l'autre n'a donné à la determination des especes organisées fossiles, dans chaque genre de couche, la rigueur devenue necessaire, depuis que les animaux connus s'elevent a un nombre si prodigieux. Tome į. P. 34. ARTICLE X. Proceedings of Philosophical Societies. ROYAL SOCIETY. ON Thursday the 25th of February the following papers were read: An account of a new micrometer, by Dr. W. Hyde Wollas ton. It consists of an instrument similar in appearance to a common telescope, with three sliding tubes. At one end there is a spherical lens, with a focus of about th of an inch; and withinth of an inch of it, there is a slit, through which objects can be seen. Instead of the object-glass there are a number of parallel wires placed beside each other, of a determinate diameter, as an object of comparison. Within are two glasses, between which the object to be measured is put. Its size is determined by comparing the magnified image with the standing wires at the extremity of the tube, and ascertaining their distance from the eye when both appear of a size: 16 inches distance in the instrument corresponds to a wire of o of an inch in diameter, and 8 inches to one of th of an inch. A paper on the winter solstice, by Mr. Pond, Astronomer Royal. He found the obliquity of the ecliptic at the summer solstice to be 23° 27′ 51.5", and at the winter solstice 23° 27′ 47.37". The difference he conceives to depend upon refraction. He is endeavouring at present to ascertain whether Dr. Bradley's allowance for refraction be correct. A paper on the black matter in the glands of the lungs in old persons, by Dr, Pearson. The lungs are at first light coloured; but they become mottled about the age of 20, gradually increase in darkness, and in old persons are nearly black. Dr. Pearson examined the cause of this change, and found it owing to a quantity of charcoal contained in the glands. This charcoal, he conceives, is taken in with the breath, suspended in the air. He accounts for its absence in young persons, and in brute animals, |