SECT. XIV. CAPILLARY ATTRACTION. 109 



will be a hemisphere whose diameter is also the diame- 

 ter 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 action is insensible at sen- 

 sible distances, and that it is only the thinnest possi- 

 ble 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 ex- 

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

 whatever be their thickness or density, because the mi- 

 nute coating of moisture is sufficient to remove the in- 

 ternal column of mercury 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 capillary 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 extremely 

 small space at its surface. Every indefinitely thin layer 

 of a liquid is compressed by the liquid above it, and sup- 

 ported by that below. Its degree of condensation de- 

 pends upon the magnitude of the compression force ; 

 and as this force decreases rapidly toward 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 

 K. 



