270 FOURTH REPORT — 1834. 



Ciliated are found to be sufficiently accordant with the same 

 quantities determined experimentally. 



The theory conducts also to the following result : " The linear 

 dimensions of similar drops of different fluids depending from 

 a horizontal surface vary in the same ratio as the heights of 

 ascent of the respective fluids against a vertical surface, or as 

 the square roots of ascent in a given tube." 



In explaining the instances of apparent attractions and re- 

 pulsions treated of by Monge, Dr. Young shows that in the two 

 cases of attraction, the force which urges the planes towards 

 each other varies inversely as the square of the interval between 

 them, because it varies once inversely as the distance on account 

 of the increase of curvature of the fluid surface as the interval 

 diminishes, and again inversely as the distance by reason of the 

 increase, proportional to the ascent of the fluid, of the surface on 

 which the capillary action is exerted. With respect to the 

 third case, that of repulsion, which is omitted in Laplace's 

 treatise, he remarks, that " the repulsion of a wet and dry body 

 does not appear to follow the same proportion, for it by no 

 means approaches to infinity on the supposition of perfect con- 

 tact : its maximum is measured by half the sum of the elevation 

 and depression on the remote sides of the substances, and as the 

 distance increases, this maximum is only diminished by a quan- 

 tity which is initially as the square of the distance." 



The strong cohesion of two solids produced by the interposi- 

 tion of a small quantity of a fluid, which wets them, between 

 their plane surfaces, is sufficiently accounted for by the great 

 curvature, arising from the proximity of the surfaces, of the 

 outer boundary of the interposed fluid, which is everywhere con- 

 cave like the rim of a pulley. This curvature corresponds to a 

 force which would be capable of sustaining a great elevation of 

 the fluid ; but in this instance the force is not exerted in sup- 

 porting the fluid, but acts by reason of the mutual attraction 

 between the solid and fluid, on the plane surfaces of the solids, 

 drawing them together. If the solids were wholly immersed in 

 the fluid, no such cohesion would take place. On the same prin- 

 ciple, if fluid be interposed between two solids which it is quite 

 incapable of wetting, so that its boundary is everywhere convex, 

 the force due to the convexity, being directed inwards, would 

 present a strong opposition to any force tending to make the 

 planes approach each other. 



After showing, in the manner exhibited above, that a theory 

 founded on the two assumptions of a uniform tension of curved 

 fluid surfaces, and a constant angle of contact of the surface of 



