6 ON CAPILLARY ACTION. 



Picssure pro- "by that of the parts removed ; and the pressure will there- 

 curvature. ^^^^ ^^ every case be proportional to the curvature. 

 A drop of fluid Heiice if we imagine a drop of a fluid to be perfectly in- 

 being pert ecty sulated, it is evident, that the superficial parts on one side 

 ' of the drop must press the included fluid towards the other 

 side, and must consequently be pressed back in an equal 

 degree, so that at the circumference of the circle supposed, 

 to divide the drop, the surface must be stretched by the 

 the tension of whole of this force, reduced only to a single direction ; and 



us surface wil ^]^f.y^ must therefore be a uniform tension of the surface, 

 be uniform m, , , . , i i «• , 



1 he only case which can be supposed to anord an exception 



Exception to this demonstration, is that of the surface of a liquid ter- 

 where the sur- minated on each side bv a solid of precisely half the den- 

 faceofthefluid . , , -^ • r- v^.i i . , , 



is bounded on ^ity : but it is ot little consequenceiwhat may be the result 



each side by a of such a combination, since it is scarcely possible, that it 

 the density.^ should ever be observed in nature. If it were not true, that 

 Tension of ^^^^ surfaces of liquids are stretched by a uniform force, it 

 fluid surfaces wou Id follow, that a coik, wetted on one side and greased 

 Hjus e eq ^^ ^^^ other, would continue for ever to move, on the sur- 

 face of a large reservoir, towards the wetted side. 

 Hence the an- The angle of contact of a solid and a fluid, of given den- 

 gle of contact sity, may be deduced from the law of equable tension, when 



of a solid and a /ir i i • i.- r i ^ 



fluid may be ^'^*^^ established, in a very satislactory manner. Conceive 



deduced. a body, of the density of the solid only, to extend through 



the substance of the solid and fluid ABC (Fig. 6) ; the 

 attraction' of its surface will then urge the angular particle 

 in the direction B D, with a force which is to the whole ten- 

 sion as B D to half A B ; then a substance equal in density 

 to the difference of the solid and the fluid, being superadded 

 to the wedge C B E, will draw the particle in the direction 

 B F with a force B F : now in order that the forces in the 

 directions B D and B F may produce a result B G, capable 

 of being completely counteracted by the perpendicular at- 

 traction of the surface A E, they must be proportional to 

 BD and D G, and the density of the additional portion 

 C B E must be to that of the solid as D G to B F, or to 

 D C or A D, and the whole density of the liquid C B E to 

 that of the solid as A G to A D, that is, by similar trian- 

 gles, as A E to A H, which is the versed sine of the angle 

 A ]5 C, A E being the diameter. 



Mr. 



