110 CAPILLARY ATTRACTION. SHCT. XIV. 



density of the capillary surface round the edges from the 

 attraction of the tube. 



The direction of the resulting force determines the 

 curvature of the surface of the capillary column. In 

 order that a liquid may be in equilibrio, the force re- 

 sulting from all the forces acting upon it must be per- 

 pendicular to the surface. Now it appears that as glass 

 is more dense than water or alcohol, the resulting force 

 will be inclined toward the interior side of the tube ; 

 therefore the surface of the liquid must be more ele- 

 vated at the sides of the tube than in the center in order 

 to be perpendicular to it, so that it will be concave as in 

 the thermometer. But, as glass is less dense than mer- 

 cury, the resulting force will be inclined from the interior 

 side of the tube (N. 170), so that the surface of the ca- 

 pillary column must be more depressed at the sides of 

 the tube than in the center, in order to be perpendicular 

 to the resulting force, and is consequently convex, as 

 may be perceived 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 

 downward, 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, ac- 

 cording to the relative densities of the plates and the 

 liquid (N. 171) ; and the phenomena will be exactly 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 grad- 

 ually 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. 



