Dr. Young’s principal objects in this paper are to reduce the phenomena of the capillary action of fluids to the general law of an equable tension of their surfaces; to investigate the properties of the curves resulting from this law; to determine the magnitude of the apparent adhesion of solids to fluids, and the cohesion of moistened solids; and to show in what manner the law itself is probably derived from the fundamental properties of matter. Dr. Young observes, that a fluid which is not capable of wetting a given solid, forms with it an angle of contact which is constant in all circumstances; that the curvature of the surface of a fluid, or the sum of the curvatures, where the curvature is double, is always proportional to the elevation or depression with respect to the general surface, and that the curve itself admits, in all cases, an approximate delineation by mechanical means, but is not always capable of being easily subjected to calculation. When, however, the curvature is simple, the direction of the surface, at any given height, admits a correct determination. Hence the elevation of a fluid in contact with a given surface, whether vertical, horizontal, or inclined, is deduced from its ascent between plates, or in a tube, of the same substance; and the result is shown to agree with experiments. Thus, taking 1/25-th of an inch for the diameter of a tube, in which water rises to the height of an inch, it is inferred that the apparent adhesion of water, to a square inch of any horizontal surface capable of being wetted by it, must be 50½ grains, which is only half a grain more than the result of Taylor’s experiments. The adhesion of alcohol, and of sulphuric acid, as measured by Achard, are also found to agree with the ascent of those fluids in capillary tubes. Lord Charles Cavendish’s table of the depression of mercury in barometer tubes, is compared with the same principles by means of diagrams constructed for each particular case; and the apparent adhesion of the surface of mercury to glass, as well as the depth of a portion of mercury spread on a plate of glass, is deduced from these measures, and is shown to agree with experiments. The observations of Morveau, on the attraction of the different metals to mercury, are also discussed; and some remarks are made on the magnitude of drops of various substances.