Page:Elementary Text-book of Physics (Anthony, 1897).djvu/115

§ 86] to each abscissa is inversely as the distance between the plates at that point, and the elevations are therefore inversely as the abscissas: hence the product of any abscissa by its corresponding ordinate is a constant. The extremities of the ordinates then mark out a rectangular hyperbola referred to its asymptotes.

86. Movements of Solids.—In certain cases the action of the capillary forces produces movements in solid bodies partially immersed in a liquid. For example, if two plates, which are both either wetted or not wetted by the liquid, be partially immersed vertically, and brought so near together that the rise or depression of the liquid due to the capillary action begins, then the plates will move towards one another. In either case this movement is explained by the inequality of pressure on the two sides of each plate. When the liquid rises between the plates, the pressure is zero at that point in the column which lies in the same plane as the free external surface. At every internal point above this the molecules of the liquid are in a state of negative pressure or tension, and the plates are consequently drawn together. When the liquid is depressed between the plates, they are pressed together by the external liquid above the plane in which the top of the column between the plates lies. When one of the plates is wetted by the liquid and the other not, the plates move apart. This is explained by noting, that, if the plates be brought near together, the convex surface at the one will meet the concave surface at the other, and there will be a consequent diminution in both the elevation and the depression at the inner surfaces of the plates. The elevation and depression at the outer surfaces remaining unchanged, there will result a pull outwards on the wetted plate and a pressure outwards on the plate which is not wetted; and they will consequently move apart. Laplace showed, however, as the result of an extended discussion, that, though seeming repulsion exists between two plates such as we have just considered, yet, if the distance between the plates be diminished beyond a certain value, this repulsion changes to an attraction. This prediction has been completely verified by the most careful experiments.