62f lECTURE L. 



where, and therefore concave; consequently the fluid,, mu,st ascend until, a[^ 

 arrives at a position capabb v4 aftording an equilibrium i,u this manner: if^ 

 on the contrary, the attractu-e power of the solid be wealter, the- liquid will 

 descend, and its surface will be convex. (Plate;XX^IX. Fig. S32.) 



■ if . -■'■. 

 It may also be shown, that if the attractive power of, the solid be equal to 

 that of the liquid, or still greater, it will be wetted by the liquid, which will 

 rise until its surface acquires the same direction with that of the solid; and 

 in other cases the angle of contact will be greater in proportion as the solid 

 is less attractive. These conclusions are obtained by comparing the jconunon 

 surface of the liquid and solid with the surface of a single liquid, of which 

 the attractive power is equal only to the difference of th^, respective powers of 

 the substances concerned; and the comparison is equally applicable to the 

 contact of two liquids of different densities. 



The magnitude of the superficial cohesion or contractility of a liquid may 

 be expressed, for a certain extent, by a certain weight; thus every inch of 

 the surface of water is stretched each way by a force equal to the weight 

 of the hundredth part of a cubic inch of water, or to two grains and a 

 half: and for each inch of the surface of mercury, the force is equi- 

 valent to 17 grains, which is the weight of -^-^ of a cubic, inch of. mer- 

 cury. Thus if a solid of any form, of which the surfaces are vertical, 

 and which is capable of being wetted by either of these fluids, be immersed 

 ■into a reservoir containing it, the fluid will be elevated around it to such a 

 height, that 24: or 17 grains, for each inch of the circumference of the solid,, 

 will retnain above the general level of the reservoir, the surface assuming 

 nearly the same form as a very long and slender elastic rod, fixed horizontally 

 at one end, and bearing a large weight at the other. (Plate XXXIX. Fig. 

 534.) 



The elevation of^ the summit of an extended surface of water, in contact 

 with the flat and upright surface of a solid which is wetted by it, is one ^ 

 seventh of an inch: but when two such surfaces, for instance, two plates of 

 glass, are brought near to each other, the elevation of the water between them 

 must be greater than this, in order that each inch of the line of contact may 

 support its proper weight : thus, if the distance were one fiftieth of an inch, 



