]28 CAPILLARY ATTRACTION. [SECT. xiv. 



sphere whose diameter is also the diameter of the tube 

 (N. 168). The elevation or depression of the same liquid in 

 different tubes of the same matter is in the inverse ratio of 

 their internal diameters (N. 169), and altogether independent 

 of their thickness ; whence it follows that the molecular ac- 

 tion is insensible at sensible distances, and that it is only 

 the thinnest possible film of the interior surface of the tubes 

 that exerts a sensible action on the liquid. So much indeed 

 is this the case, that, when tubes of the same bore are com- 

 pletely wetted with water throughout their whole extent, 

 mercury will rise to the same height in all of them, whatever 

 be their thickness or density, because the minute coating of 

 moisture is sufficient to remove the internal column of mer- 

 cury beyond the sphere of attraction 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 ab- 

 struse subjects, has proved that the phenomena of capillary 

 attraction depend upon a rapid decrease in the density of the 

 liquid column throughout an extremely small space at its 

 surface. Every indefinitely thin layer of a liquid is com- 

 pressed by the liquid above it, and supported by that below. 

 Its degree of condensation depends upon the magnitude of 

 the compression 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 



