114 CAPILLARY ATTRACTION. SECT. XIV. 



of the tube, and to supply the place of a tube by its own capillary 

 attraction. The forces which produce the capillary phenomena 

 are the reciprocal attraction of the tube and the liquid, and of 

 the liquid particles on one another ; and, in order that the capil- 

 lary column may be in equilibrio, the weight of that part of it 

 which rises above or sinks below the level of the liquid in the 

 cup must balance these forces. 



The estimation of the action of the liquid is a difficult part of 

 this problem. La Place, Dr. Young, and other mathematicians, 

 have considered the liquid within the tube to be of uniform 

 density ; but M. Poisson, in one of those masterly productions in 

 which he elucidates the most abstruse subjects, has proved that 

 the phenomena of capillary attraction depend upon a rapid 

 decrease in the density of the liquid column throughout an ex- 

 . tremely small space at its surface. Every indefinitely thin layer 

 of a liquid is compressed by the liquid above it, and supported 

 by that below. Its degree of condensation depends upon the 

 magnitude of the compressive force ; and, as this force decreases 

 rapidly towards the surface, where it vanishes the density of the 

 liquid decreases also. M. Poisson has shown that, when this 

 force is omitted, the capillary surface becomes plane, and 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 towards the interior side of the tube ; there- 

 fore 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. 174), so that the surface 

 of the capillary column must be more depressed 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 



