4 ON CAPILLARY ACTION. 



equal: for, if an imaginary separation. be made in tlie sub- 

 stance in any direction A C, it is evident, that the cohesive 

 force, tending to bring all the particles of the two segments 

 togetlicr, nniist be ex\uix\ to the repulsive force, which prevent;s 

 their nearer approach ; and, into whatever portions the co- 

 hciiive forces of the wedges be supposed to be divided, it is 

 obvious, that for each of these, as for example, the mutual 

 actions of the particles situate at II and I, an equivalent 

 may be found in the repulsive force K L, exerted between 

 the particles whicli are similarly situate within the sphere 

 of repulsion: consequently, the whole result must be not 

 only equal, but also parallel: so that, if the wedge F C G 

 be considered as the termination of a vertical column, the 

 eflect of the wedge D C E, or of D C G, will have no ten- 

 Thfi counter- dency either to elevate or to depress that column. The only 



tctiou not ab- ^^^^ ^^ pt^pfcct counteraction will be, that the parts nearest 

 •olutely per- ' _ *^ . 



tect. the wedge will be urged more downwards by the repulsive 



force, and the remoter parts more upwards by the cohesive 



force. In order to understand the effect of a combination 



Effect of a of such actions where the surface is curved, let us suppose 



combination of ^^^^ superficial particles to be situate at the angles of a po- 

 such actions. it n i 



lygon, A B C D E F G H, (Fig. 4) and the repulsive force 



to extend only to the two nearest particles, one on each 

 side, while the'coliesive force is so distributed, as to have its 

 general result directed to the next particle but one: it will 

 then be necessary, in order that there may be an equilibrium 

 between the ix)!ccs tending to separate and to unite any two 

 particles D and E in the direction of the surface, that the 

 cohesive forces in the directions D F, EC, be represented 

 by D I and I K, while D E represents the repulsive force: 

 then the forces acting on D being represented by CD, 

 £ D, L D, and I D, it is evident, that the parts of these 

 forces which tend to urge the particle D to and from the 

 line C E, are precisely equal, so that this particle will re- 

 main perfectly in equilibrium, without occasioning any 



Dr Yonng\s pressure on the stratum within it. It is supposed in Dr. 



supposition. Young's reasoning on this subject, that the repulsive and 

 cohesive forces acting on each particle are either accurately 

 or very nearly equal ; but this supposition, although it ap- 

 pears 



