The cohesion of fluids is beautifully shewn in a phe nomenon, which is the very reverse of the formation of a drop, and which was first observed by Dr Brewster. If we take a phial, with a wide mouth, half filled with Canada balsam, and allow the balsam to flow to the mouth of the phial and fill it up, then when the phial is placed on its bottom, a fine transparent film of balsam will be seen extending over the mouth of the phial. If we now take a piece of slender wire, and touch the film near the mid dle, so as to tear away a little part of it, the remaining part of the film which has been elevated by this force will descend to its level position, and the ragged aperture from which the balsam as been torn will be seen to assume a form per fectly circular, having its edge in a slight de gree thickened, like a circle with a raised margin turned out of a piece of wood. This fine circular aperture grows wider and wider, and continues to preserve its circular lorm till the mouth of the phial is again opened.
The following curious experiment, which was perform ed by Dr Brewster, is intimately connected with the sub ject of capillary attraction. Above a vessel A1NOP, Fig. 6. nearly filled with water, a convex lens LL was placed at the distance of the 10th of an inch, and rays R, R, R, were incident upon its upper surface. The focus of these rays was at F, a little beyond the bottom of the vessel, so nett a circular image of the luminous object was seen on the bottom of the vessel, having AB for its diameter. if the lens is now made to descend gradually towards the sur face of the water, and the eye kept steadily upon the luminous image AB, a dark spot will be seen at 0 in the centre of AB, a little while before the lens attracts and eleNates the water AIN. Sometimes this spot may be seen playing back and forwards by the slight motion of the nand, so that the lens can even be withdrawn from the fluid surface without having actually touched it. In gene ral, however, the sudden rise of the water to the lens fol lows the appearance of the black spot. \Vhen the water is in contact with the glass, the focus of the rays R, R is now transferred to f, and the circular image on the bottom is now a b, and the intensity of the light in this circle is to that in the circle AB, as : a Now it is obvious, that the darkish spot at 0 is just the commencement of the transference of the focus from F to f; or when the dark spot is produced, the progress of the rays is the same as if the focus were transferred to f. This remarkable effect may arise front two causes. f. The approach of the lens to the surface MN, may occasion a depression o n in the surface of the fluid ol the same curvature as L / L, which would have the effect of transferring the locus from F to f. This depression may be produced by a film ol air adhering to the lens, in the same manner as the depression is produced on the surface of alcohol, by a drop of alcohol rolling over it, as shewn in Fig. 5, or by sonic other repulsive force with which we are unac quainted. 2. The transference of the focus from F to f, may be occasioned by the optical contact of the glass and water taking place at a greater distance from the lens than the distance at which the capillary attraction commences.
For if the surfaces 11, n, at a certain distance, act upon the rays of light as if they were one surface, then it is manifest that a dark spot ought to appear at 0, if this distance, is less than that at which capillary attraction begins.
In the first supplement published by M. Ic Comte La Place, his method of considering the phenomena was founded on the form of the surface of the fluid in capillary spaces, and on the conditions of equilibrium of this fluid in an infinitely narrow canal, resting by one of its extre mities upon this surface, and by the other on the horizon tal surfaces of an indefinite fluid, in which the capillary tube was immersed. In his second supplement, he has examined the subject in a much more popular point of view, by considering directly the forces which elevate and depress the fluid in this space. By this means he is con ducted easily to several general results, which it would have been difficult to obtain directly by his former method. Of this method we shall endeavour to give as clear a view as possible.
Let AB, Fig. 7. be a vertical tube whose sides are per pendicular to its base, and which is immersed in a fluid that rises in the interior of the tube above its natural level. A thin film of fluid is first raised by the action of the sides of the tube ; this film raises a second film, and this second film a third film, till the weight of the volume of fluid raised exactly balances all the forces by which it is actu ated. Hence it is obvious, that the elevation of the column is produced by the attraction of the tube upon the fluid, and the attraction of the fluid for itself. Let us suppose that the inner surface of the tube AB is prolonged to E, and after bending itself horizontally in the direction ED, that it assumes a vertical direction DC ; and let us sup pose the sides of this tube to be so extremely thin, or to be formed of a film of ice, so as to have no action on the fluid which it contains, and not to prevent the reciprocal action which takes place between the particles of the first tube AB and the particles of the fluid. Now, since the fluid in the tubes AE, CD is, in equilibrium, it is obvious, that the excess of pressure of the fluid in AE is destroyed by the vertical attraction of the tube and of the fluid upon the fluid contained in AB. In analyzing these different attractions, M. La Place considers first those which take place under the tube AB. The fluid column BE is attract ed, 1. by itself ; 2. by the fluid surrounding the tube BE. But these two attractions are destroyed by the similar at tractions experienced by the fluid contained in the branch DC, so that they may be entirely neglected. The fluid in BE is also attracted vertically by the fluid in AB ; but this attraction is destroyed by the attraction which it exercises in the opposite direction upon the fluid in BE, so that these balanced attractions may likewise be neglected. The fluid in BE is likewise attracted vertically upwards by the tube AB, with a force which we shall call' Q, and which con tributes to destroy the excess of pressure exerted upon it by the column BF raised in the tube above its natural level.