CAPILLARY ACTION. 545 



heading " Fluids, Elevation of," in the supplement to the fourth edition of the 

 Encyclopedia Britannica, published in 1819. 



In 1804 Thomas Young* founded the theory of capillary phenomena on 

 the principle of surface-tension. He also observed the constancy of the angle 

 of contact of a liquid surface with a solid, and shewed how from these two 

 principles to deduce the phenomena of capillary action. His essay contains the 

 solution of a great number of cases, including most of those afterwards solved 

 by Laplace, but his methods of demonstration, though always correct, and often 

 extremely elegant, are sometimes rendered obscure by his scrupulous avoidance 

 of mathematical symbols. Having applied the secondary principle of surface- 

 tension to the various particular cases of capillary action, Young proceeds to 

 deduce this surface-tension from ulterior principles. He supposes the particles 

 to act on one another with two different kinds of forces, one of which, the 

 attractive force of cohesion, extends to particles at a greater distance than 

 those to which the repulsive force is confined. He further supposes that the 

 attractive force is constant throughout the minute distance to which it extends, 

 but that the repulsive force increases rapidly as the distance diminishes. He 

 thus shews that at a curved part of the surface, a superficial particle would 

 be, urged towards the centre of curvature of the surface, and he gives reasons 

 for concluding that this force is proportional to the sum of the curvatures of 

 the surface in two normal planes at right angles to each other. 



The subject was next taken up by Laplace t. His results are in many 

 respects identical with those of Young, but his methods of arriving at them 

 are very different, being conducted entirely by mathematical calculations. The 

 form into which he has thrown his investigation seems to have deterred many 

 able physicists from the inquiry into the ulterior cause of capillary phenomena, 

 and induced them to rest content with deriving them from the fact of surface- 

 tension. But for those who wish to study the molecular constitution of bodies 

 it is necessary to study the effect of forces which are sensible only at insen- 

 sible distances ; and Laplace has furnished us with an example of the method 

 of this study which has never been surpassed. Laplace investigates the force 

 acting on the fluid contained in an infinitely slender canal normal to the 

 surface of the fluid arising from the attraction of the parts of the fluid out- 



* Essay on the "Cohesion of Fluids," Philosophical Transactions, 1805, p. 65. 

 t Mecanique Celeste, supplement to the tenth book, published in 1806. 

 vol.. II. 69 



