COHESION OF FLUIDS. ^^ 



a similar opinion^ has shown in what manner the principle may General piinci? 

 be deduced from the doctrine of attraction, but his demonstra- ?^^oaof fluids' 

 tion is complicated, and not perfectly satisfactory ; and in ap- 

 plying the law to the forms of drops, he has neglected to coia- 

 sider the very material effects of the double curvature, -which 

 is evidently the cause of the want of a perfect coincidence of 

 his experiments with his theory. Since the time of Segncr, 

 little has been done in investigating accurarely and in detail 

 the various consequences of the principle. 



It will perhaps be most agreeable to -the experimental philo- 

 Josopher, although less consistent with the strict course of logi» 

 cal argument, to proceed in the first place to the comparison 

 of this theory with the phenomena, and to inquire afterwards 

 for its foundation in the ultimate properties of matter. But it 

 is necessary to premise one observation, which appears to be 

 new, and which is equally consistent with theory and with ex- 

 periment ; that is, that for each combination of a solid and a 

 fluid, there is an appropriate angle of contact between the sur- 

 faces of the fluid, exposed to the air, and to the solid. This 

 angle, for glass and water, and in all cases where a solid is 

 perfectly wetted by a fluid, is evanescent: for glass and mer- 

 cury, it is about 140^ in common temperatures, and when the 

 jnercury is moderately clean. 



II. Form of the Surface of a Fluid, 

 It is well known, and it results immediately from the com- Form of the 

 position of forces, that where a line is equally distended, the fl"'-d *^^ ^n^di- 

 force that it exerts, in a direction perpendicular to its own, is fied by the co* 

 directly at its curvature; and the same is true of a surface o^ parts" &c.*** 

 simple curvature ; but where the curvature is double, each 

 curvature has its appropriate effect, and the joint force must 

 be as the sum of the curvatures in any two perpendicular direc- 

 tions. For this sum is equal, whatever pair of perpendicular 

 directions may be employed, as is QCisily shown by calculating 

 the versed sines of t\\ o equal arcs taken at right angles in the 

 surface. Kow Avhcn the surface of a fluid is convex exter- 

 nally, its tention is produced by the pressure of the particles 

 of tlie fluid within it, arising from their own weight, or from, 

 that of the surrounding fluid ; but when the surface is con- 

 ceive, the tension is employed in counteracting th« pressure of 

 L 2 Ihe 



