Chap, xxii.] CAPILLARITY. 243 



the ascent. The surface tension of the liquid is in this 

 case increased ; but, by the law of equal transmission 

 of pressure, the increased pressure within the tube 

 cannot be permitted to remain. Consequently, the 

 liquid falls in the tube till it is depressed so far below 

 the level of the outer liquid, that the column of liquid 

 representing the difference would exert a pressure equal 

 to the increased pressure produced within; and so 

 equilibrium is restored. 



In the case of the convex meniscus the depression 

 of the liquid is in the inverse ratio to the diameter of 

 the tube. The result is, however, in this case affected 

 by the nature of the material forming the tube. Thus, 

 the depression of mercury in a tube of iron is greater 

 than the depression of mercury in a tube of platinum 

 of the same bore. It also varies with the nature of 

 the liquid and the temperature, diminishing with an 

 increasing temperature. 



Suppose, then, that a vertical glass tube, wide 

 enough to permit the neglect of capillary phenomena, 

 communicates with a vertical capillary tube, and that 

 water is poured into the wide tube. The water will 

 rise in the capillary tube considerably above the level 

 of the water in the wide tube, because of the diminution 

 of hydrostatic pressure by the force of adhesion. If a 

 similar tube contain mercury, then the mercury w T ill 

 be depressed in the capillary tube considerably below 

 the level of that in the wide tube, because of the in- 

 creased hydrostatic pressure. 



Capillary phenomena of a similar character are 

 observed if two plane surfaces be brought near 

 enough to one another, whether parallel or inclined 

 to one another. If they are inclined to one 

 another, then a small quantity of a liquid that wets 

 the surface placed between them will move from the 

 wide to the narrow end ; and if the liquid does not 

 wet them it will move in the opposite direction. 



