ON COHESION. 477 



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

 is equal only to the difference of the respective powers of the substances 

 concerned. In this manner it may 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 that, in other cases, the angle of contact will be 

 greater, in proportion as the solid is less attractive. A similar comparison 

 is also 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 equivalent to 17 

 grains, which is the weight of ^^-y of a cubic inch of mercury. 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 con- 

 taining it, the fluid will be elevated around it to such a height that 2^ or 

 17 grains [respectively], for each inch of the circumference of the solid, 

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

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

 tally 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 w^ater, 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 con- 

 tact may support its proper weight : thus, if the distance were one fiftieth 

 of an inch, the elevation would be a whole inch ; and if the distance were 

 smaller than this, the elevation would be greater in the same proportion ; 

 so that when two plates are placed in such a manner as to touch each other 

 at one of their upright edges, the outline of the water raised between them 

 assumes the form of a hyperbola. (Plate XXXIX. Fig. 535.) 



The weight supported by the cohesion of the water in a tube may be 

 determined, in a similar manner, from the extent of the circumference ; the 

 height being an inch in a tube one twenty fifth of an inch in diameter, or 

 as much greater as the diameter of the tube is smaller : and in a tube 

 wetted with mercury the height would be half as great. It is obvious that 

 if the lower part of the tube be either contracted or dilated, the height of 

 the fluid will remain unaltered, while its weight may be varied without 

 limit ; for the hydrostatical pressure on the surface is the same, in both 

 these cases, as if the diameter of the tube were equal throughout its length. 

 (Plate XXXIX. Fig. 536.) 



The attractive force of glass to mercury is less than half as great as the 

 mutual attraction of the particles of mercury, and the surface of mercury 

 in a dense glass vessel becomes, therefore, convex and depressed ; the angle 

 of contact being about 140, and the depression one 17th of an inch. 

 Between two plates of glass, the depression of mercury is an inch when 



