SECT. XIV. CAPILLARY ATTRACTION. 115 



in the mercury of the barometer when rising. The absorption of 

 moisture by sponges, sugar, salt, &c., are familiar examples of 

 capillary attraction. Indeed the pores of sugar are so minute, 

 that there seems to be no limit to the ascent of the liquid. 

 Wine is drawn up in a curve on the interior surface of a glass ; 

 tea rises above its level on the side of a cup ; but, if the glass 

 or cup be too full, the edges attract the liquid downwards, and 

 give it a rounded form. A column of liquid will rise above or 

 sink below its level between two plane parallel surfaces when 

 near to one another, according to the relative densities of the 

 plates and the liquid (N. 175) ; and the phenomena will be ex- 

 actly the same as in a cylindrical tube whose diameter is double 

 the distance of the plates from each other. If the two surfaces 

 be very near to one another, and touch each other at one of their 

 upright edges, the liquid will rise highest at the edges that are 

 in contact, and will gradually diminish in height as the surfaces 

 become more separated. The whole outline of the liquid column 

 will have the form of a hyperbola. Indeed, so universal is the 

 action of capillarity, that solids and liquids cannot touch one 

 another without producing a change in the form of the surface of 

 the liquid. 



The attractions and repulsions arrising from capillarity present 

 many curious phenomena. If two plates of glass or metal, both 

 of which are either dry or wet, be partly immersed in a liquid 

 parallel to one another, the liquid will be raised or depressed 

 close to their surfaces, but will maintain its level through the 

 rest of the space that separates them. At such a distance they 

 neither attract nor repel one another ; but the instant they are 

 brought so near as to make the level part of the liquid disappear, 

 and the two curved parts of it meet, the two plates will rush 

 towards each other and remain pressed together (N. 176). If 

 one of the surfaces be wet and the other dry, they will repel one 

 another when so near as to have a curved surface of liquid 

 between them ; but, if forced to approach a little nearer, the re- 

 pulsion will be overcome, and they will attract each other as if 

 they were both wet or both dry. Two balls of pith or wood 

 floating in water, or two balls of tin floating in mercury, attract 

 one another as soon as they are so near that the surface of the 

 liquid is curved between them. Two ships in the ocean may be 



