ifusion of Liquids. 327 



point showed that for velocities up to 70 cubic centims. per 

 hour the amount diffused was independent of this factor, but 

 for velocities above this it rapidly decreased. It w r as not until 

 the indigo- water was resorted to that this action was explained. 

 It was then found to be as follows : — For ordinary velocities, 

 the reflecting surface occupies in the beginning the position 

 ef nearly, as shown above ; and this is true whether the velo- 

 city be 30 or 70 cubic centims. per hour. However, if it be 

 increased to 80 cubic centims., a change takes place : the 

 reflecting surface no longer remains stationary at ef, but is 

 pushed downward in i until finally the salt-solution is entirely 

 expelled from this part of the apparatus, and blue particles 

 ascend outside of i to the free surface above. The cause of 

 this is evident. The water now flows in at w faster than it 

 can be discharged at b, and it must make a place for itself by 

 passing through i into the solution outside. It thus appears 

 that the narrow opening at b exerts a rather great capillary 

 resistance, which makes itself manifest as soon as the velocity 

 of the water reaches a certain limit. The salt being expelled 

 from i, allows the blue water to stream in visible currents out- 

 ward and upward ; and these currents, instead of permitting 

 the diffusing particles below gli to pass into i, carry them 

 along upwards on its outside. This continues, of course, until 

 the increasing height of the liquid column in the beaker is 

 capable of holding the water in equilibrium at the level g h. 

 An undisturbed diffusion can then commence again ; but some 

 time must elapse before a condition of constancy is attained 

 in i; and until this is the case the amount diffused must appear 

 too small. 



Something similar is observed when, instead of increasing 

 the velocity of the flowing water, the capillary resistance at b 

 is increased. Indeed, if this increase be but slight, the action 

 is exactly the same as in the above case ; but if the increase 

 of resistance be greater, as when, for instance, the inside of 

 the tube b is covered for a short distance with a thin film of 

 wax, the action is as follows: — At first no liquid at all drops 

 from b, as a convex meniscus is formed which exerts a pressure 

 in an opposite direction. As a consequence the solution is 

 expelled from i, the level moves upward in w and in the beaker, 

 until the excess of pressure thus produced is sufficient to over- 

 come the resistance at b. As soon as this limit is reached the 

 action is entirely reversed : the water now commences to flow, 

 and drops out quite rapidly ; the level sinks in w and in the 

 beaker ; and the surface of separation rises in i. This continues 

 until the excess of pressure has entirely disappeared , when the 

 flow suddenly stops at b, and does not recommence until again 



