I/1. Hauy suspended, by a very fine thread, a small square plate of talc, so that the lower part of it was immersed in the water. Into the same vessel, at the distance of some centimetres, he immersed the lower part of a parallelopip ed of ivory, So that one of its faces was parallel to the plate of talc. The ivory was made to advance towards the talc in this state of parallelism, and was stopped at short inter vals, in order to chew that the effect of the motion commu nicated to the fluid was insensible in the experiment. When the parallelopiped of ivory, moving with great slowness, approached very nearly to the talc, the latter moved 'sud denly into contact with the ivory. In separating the two bodies, the ivory was wetted to a certain height above the level ; and in repeating the experiment before wiping the ivory, the attraction commenced sooner, and sometimes ex hibited itself at the very first, without being preceded by any sensible repulsion. This experiment was repeated se veral times with the same result.
Another series of phenomena, which indicate apparent attraction and repulsion, are seen in the motion of small lighted wicks when swimming in a basin of oil, and in time motion of camphor upon the surface of water. Although these phenumena arc not produced by capillary attraction, yet we shall give a short account of them at present, on ac count of their general similarity.
The phenomena of lighted wicks were carefully observ ed, and minutely described, by Professor Wilson of Glas gow, in the Transactions of the Royal Society of Edinburgh. His Hydrostatical Lamp, as he calls it, consists of a small circular disc of common writing paper, about 11 of an inch in diameter, having about a quarter of an inch of soft cot ton thread rising up through a puncture in the middle of the disc, to answer the purpose of a wick. If this wick is lighted, and the disc placed in a shallow glass vessel, fill ed with pure salad oil, it will immediately sail briskly for ward in one direction till it meets the side of the vessel, and will afterwards take a circular course, always bearing up to the sides, and will thus perform many revolutions. The circulation is sometimes from right to left, and sometimes from left to right. When the wick is placed out of the centre of the Osc, it will sail to that part of the disc which is farthest from the wick, and if the disc is made of an oval form, and the wick placed in one of its foci, the disc will sail in the direction of the nearest extremity of the trans verse axis. Dr Wilson observed a very active repulsion between the stem of the disc and the oil of the surface con tiguous to it. When fine charcoal dust was scattered
around the disc, it left behind it a diverging wake clear of all dust. Other such as oil of turpentine, ether, al cohol, or any of the inflammable fluids possessing much tenuity, also double rum, melted tallow, bees wax, and rosin, exhibit the same effects when the discs float upon their surface.
Dr Wilson accounts for these phenomena in the follow ing manner : When the oil has an uniform temperature, all its parts arc in equilibrio ; but when the lamp is lighted, the oil below the disc being most heated, will ex pand itself, and will be raised above the general level, from the diminution of its specific gravity, and the unbalanced upward pressure of the fluid. The weight of the disc will therefore press down the oil, or even the weight of the oil itself will cause it to rise as it were from below the wick in a thin superficial stream. Hence Dr Wilson conceives, that this constant stream will flow most readily and copiously towards that side of the base of the lamp where the resist ance is least, or where it has the shortest way to press for wards, that is, from under the wick or flame, or the edge of the disc which is the nearest. The re-action of the stream of rarefied oil rising most rapidly and most copiously from one side of the disc, will therefore impel the lamp in the contrary direction. When the discs are soaked with oil, or when they are made of a thin plate of talc, they always sink to the bottom as soon as the flame is extinguished. If a wafer much heated is thrown upon any of the fluids above mentioned, it will immediately glide away, and continue in motion till it cools.* The singular motions of pieces of camphor floating up on the surface of water have been long observed ; but they were never completely described and explained, till Al. Venturi published, in the Memoirs presenter: to the In stitute of France, his ingenious memoir, entitled Precis de quelques experiences sur la section que des cylindres de cam phre epronvent a la surface de l'eau et rejlexions sur ks mouvemens qui accompagnent cette section.
Having cut some pieces of camphor into the shape of i small cylinders, a line in diameter, and an inch high, he fixed each of them to a base of lead, and placed them tically in plates. He then poured tt ater into the plates, till it reached about half way up each cylinder. After two or three hours the cylinders began to diminish at the place where they were cut by the surface of the water, and after about twenty-four hours they were entirely cut through in to two parts, none of which had suffered any sensible di minution.