But capillary action arises from the cohesion be tween the particles of a fluid, and the attraction that takes place between them and the solid bodies with which they are brought into contact. Experiments show that these forces continue to act so long as the state of fluidity endures ; their action is constant under the same temperature ; and they are affected in degree only by the variations of heat. In the fur ther prosecution of this inquiry, we shall therefore throw out of view the effect of temperature, and shall confine our attention to develope the conse quences of corpuscular attraction.
4. The attraction of a solid body on every particle of a fluid within the sphere of its action, is a force per pendicular to the surface of the solid. This is mani• fest from the homogeneity of the solid when its sur face is a plane ; for, on account of the uniform ar rangement of the parts, there is no reason why the attractive force should decline from the perpendicu lar to one side rather than to another. And when the solid is bounded by a curve of any kind, we may still consider the extremely small part of the surface which acts on a particle, as coinciding with the tan gent plane ; whence we may conclude that, in all cases, the attraction on every particle is perpendicu lar to the surface of the attracting body. The same thing is true in the action of transparent bodies on light. For, if the motion of a ray be decomposed into two parts, one parallel to the refracting surface, and the other perpendicular to it, the observed law of refraction implies, that the velocity of the first part will remain unchanged, while the velocity of the other part will be increased or diminished by the re fracting force.
If a smooth plate of glass be laid horizontally upon the surface of water, it is found that the glass will adhere to the water. The adhesion is not produced by the pressure of the atmosphere, for the fact is equally true in the vacuum of an air-pump. There Is therefore, evidently an attraction between the glass and the water, acting perpendicularly to the plate, and causing it to adhere to the water.
If the plate, instead of being laid horizontally up on the water, be immersed vertically in it, the part below the surface will exert the same attractions as it did in the former position. Every particle of the fluid within the sphere of action of the glass will be drawn perpendicularly towards it, and a thin coat ing of the fluid will attach itself to all the immersed surface of the plate.
S. Although the attractive force exerted by a solid body on a fluid is confined to insensible dis tances, it must still be considered as penetrating in some degree into the fluid mass. The thin
film on which it acts retains possession of all the properties of a fluid. The particles of water in contact with the glass press upon its surface ; the particles farther off press upon those nearer ; and the whole film is in a state of compression. But it is a distinguishing property of a fluid, arising from the perfect mobility of its particles, that e pressure in one direction will cause an equal pressure in all directions ; and hence we must infer that the thin film of water, at the same time that it is compressed by the direct attraction of the glass, will likewise press laterally, or will make an effort to spread itself towards every side on the surface of the plate. If the film, instead of being attracted by the plate, were pressed against its surface by a weight, the lateral pressure, estimated on a given superficial space, would be the same with the direct pressure. But, as the strata, at different. distances from the plate, are attracted in unequal degrees, the whole lateral force can be found only by summing up the lateral pressures arising from the attraction upon each stratum.
Let AB (fig. 2.) be a plate of glass on which there stands an upright vessel, or tube, containing water ; and let GH be a thin section, or elementary part of the water within the tube, parallel to the glass and so near it as to be attracted by it. Suppose that denotes the area of the section, a its distance from the plate, and da its thickness ; and let '1(a) repre sent the attraction of the whole matter of the plate upon a single particle of water placed at the dis tance a. Then, the density being constant and equal to unit., the attractive force of the plate upon the thin elementary section will be equal to T(a) x tvda : and hence the attraction of the plate upon all the water in the tube will be equal to the integral tv xfi(a).da, generated while a increases from o to be infinitely great. The expressionfi(a).da, which we may de note by K', is therefore the force with which the attraction of the glass causes the fluid to press upon a square inch of the plate, or it is the mea sure of that force. If the particles of the fluid were attracted by the matter of the plate with an intensity equal to their own cohesive force, then K' would be equal to K, that is, it would be equal to the force with which an indefinite mass of the fluid causes the superficial stratum to press inward.