REPORT ON CAPILLARY ATTRACTION. 271 



a given fluid with that of a given solid, will explain various ca- 

 pillary phsenomena, Dr. Young proceeds to derive these laws 

 from ulterior physical principles. To account for the first he 

 reasons as follows. The repulsive force which appears to act 

 uncontrolled in aeriform bodies, exists also in fluids and solids. 

 In these it is counteracted by a cohesive force. These forces in 

 fluids are so balanced, that they allow the particles to move freely 

 in all directions. In solids the cohesion is accompanied by a 

 force opposed in greater or less degree to all lateral motion, and 

 independent, as he supposes, of the cohesive force. He considers 

 it simplest to regard the cohesive force as nearly or perfectly con- 

 stant in its magnitude throughout the minute distance to which 

 it extends, and its apparent variation to be owing to the variation 

 of the repulsive force, which diminishes with the increase of the 

 distance. In the internal parts of a fluid, the two forces hold 

 the particles in equilibrium ; but wherever the surface is curved 

 or angular, it would be found by collecting the eff^ect produced 

 on a given particle at the surface, by all the particles contiguous 

 to it and lying within the sphere of its proper activity, that on 

 the above hypothesis respecting the relative variation of these 

 forces, the cohesion must necessarily prevail over the repulsion. 

 The particle will consequently be urged in the normal direction 

 towards the centres of curvature of the point at which it is situ- 

 ated, whether the surface be concave or convex ; and reasons are 

 adduced by the author for concluding that the force which urges 

 it is proportional to the curvature, if single, or to the sum of 

 the curvatures in i*ectangular directions, and consequently indi- 

 cates, as is known from Mechanics, a uniform superficial ten- 

 sion. The reasoning by which he shows this need not be in- 

 troduced here. Suffice it to say, that this result might be obtained 

 by an analysis precisely the same as that which Laplace has 

 employed at the beginning of his capillary theory, in calculating 

 the attraction experienced by a superficial particle on the suppo- 

 sition of forces sensible only at insensible distances. Laplace 

 makes no hypothesis respecting the law of force, excepting that 

 it is wholly attractive. The mathematical calculation would 

 remain the same, if the force were supposed partly attractive 

 and partly repulsive, provided the attractive force decreased less 

 rapidly with the increase of distance than the repulsive, and be- 

 came the greater of the two before it ceased to be of sensible 

 magnitude. With this alteration* in Laplace's theory, it would 



• This modification of the theory is pointed out by its author in a note at 

 p. 122 of the Bulletin de la Socu'tePhilotnatique, An 1819. 



