SECT, xiv.] CAPILLARY ATTRACTION. 129 



that the liquid in the tube will neither rise above nor sink 

 below the level of that in the cup. In estimating the forces, 

 it is also necessary to include the variation in the 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 resulting from 

 all the forces acting upon it must be perpendicular to the 

 surface. Now it appears that, as glass is more dense than 

 water or alcohol, the resulting force will be inclined to- 

 wards the interior side of the tube ; therefore the surface 

 of the liquid must be more elevated at the sides of the 

 tube than in the centre in order to be perpendicular to it, 

 so that it will be concave as in the thermometer. But, as 

 glass is less dense than mercury, the resulting force will 

 be inclined from the interior side of the tube (N. 170), so 

 that the surface of the capillary column must be more de- 

 pressed at the sides of the tube than in the centre, in order 

 to be perpendicular to the resulting force, and is consequently 

 convex, as may be perceived in the mercury of the baro- 

 meter when rising. The absorption of moisture by sponges, 

 sugar, salt, &c., are familiar examples of capillary attrac- 

 tion. 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 rela- 

 tive 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, 



