130 CAPILLARY ATTRACTION. [SECT.XIV. 



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 arising 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 sepa- 

 rates 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. 172). 

 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 repulsion will be overcome, and they will at- 

 tract 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 brought 

 into collision by this principle. But two balls, one of 

 which is wet and the other dry, repel one another as soon 

 as the liquid which separates them is curved at its surface. 

 A bit of tea leaf is attracted by the edge of the cup if wet, 

 and repelled when dry, provided it be not too far from the 

 edge and the cup moderately full ; if too full, the contrary 

 takes place. It is probable that the rise of the sap in 

 vegetables is in some degree owing to capillarity. 



