Conditions of Molecular Action. 363 



capillary tubes, in porous substances, and upon the surfaces of 

 solids, and the formation of drops ; probably also many of the 

 phenomena of cementation by means of colloid substances. 



The simplest case is perhaps that of true capillary attraction. 

 When a fluid rests in a horizontal tube open to the air at both 

 ends, and so small that cohesion of the fluid will prevent it from 

 running into drops, then all the forces acting upon the fluid are 

 equal and antagonistic, except the force of gravitation, which 

 is neutralized by the cohesion of the fluid ; the solid and the 

 liquid act and react equally throughout the whole length of the 

 column of fluid ; and at each end of this column the particles of 

 the solid which are just beyond the limits of the fluid act un- 

 perturbedly, and draw it in opposite directions : hence the po- 

 sition is one of equilibrium, and no movement takes place. 

 When, however, the tube is raised into a vertical position, the 

 attraction of gravitation tends to draw the fluid downwards ; and 

 if the molecular attraction of the walls of the tube and the cohe- 

 sive power of the fluid are not sufficient to counterbalance the 

 force of gravity, the column of liquid will descend in the tube 

 until it reaches its lower extremity. Here, 

 if the tube be small enough, it will stop, 

 and no portion exude ; for the molecular 

 attraction of the lowest part of the tube, 

 which had before been acting in the same 

 direction as gravitation, and in opposition 

 to the same molecular force at the top of 

 the fluid, now changes its direction and 

 tends to draw upwards the lower particles 

 of the fluid (as in the figure) ; and if the 

 weight of the fluid is not too great, equilibrium is again esta- 

 blished, and the column of fluid remains at rest. 



It will be seen that the molecular attraction now acting up- 

 wards both at the top and bottom of the fluid has no power to 

 cause the column of fluid to ascend ; by any upward movement 

 the lower force is at once changed in direction and draws down- 

 wards again. But now let the lower end of the tube be plunged 

 into a vessel containing the same kind of fluid: the particles 

 composing this portion of the tube now draw up successive por- 

 tions of fluid which press upward those already in the tube, and 

 thus act along with the molecular force at the top of the column, 

 and gradually raise it in the tube until its vertical hydraulic 

 pressure equals the united force of these two attractions. 



It might be supposed that in these cases of capillary attraction 

 we have what we were just now looking for, an actual measure 

 of the molecular force exerted, in the height to which the fluid 

 rises in the tube. If it were possible to learn the exact number 



