810 Dr. S. W. J. Smith on the 



The chance of formation of hydrogen will be greatest at 

 the end of the jet where it breaks into drops and suddenly 

 presents a new surface to the solution. If the jet is com- 

 pletely immersed this chance will be only a little greater 

 than at the sides ; but it will be much enhanced if the end of 

 the jet just touches the surface of the solution (level of liquid 

 at b in fig. 1). 



Thus suppose the length of the completely immersed jet 

 to be I and the velocity of efflux v. The " electrochemical " 

 forces already described will tend to equalize the distribution 

 of mercury salt in solution round the jet, although the various 

 elements of the surface have been in contact with the solution 

 for times varying between zero and l/v. The end of the jet 

 after rupture is a surface bounded by that portion of the 

 rest of the jet which has been longest in contact with the 

 solution, and the concentration of mercury salt round the end 

 will therefore be raised practically instantaneously to a 

 considerable value. 



If the jet breaks in the surface of the solution, however, 

 the electrochemical short-circuit is reduced to a minimum 

 because the sides of the jet are now practically out of con- 

 tact with the solution. 



Very little hydrogen can be produced on any element of 

 the surface even when the jet has its greatest efficiency, 

 and the difficulty of formation of extremely small bubbles 

 (owing to surface tension effects) may prevent evolution of 

 gas otherwise possible in accordance with (i. a). Suppose, 

 however, that the jet is connected to a small mercury surface 

 at rest in contact with the solution as at S (tig. 1). If there 

 is less mercury in solution round J than round S, the metal 

 will precipitate at S and enter solution at J. This action 

 will continue as long as the concentration of Hg in solution 

 round S exceeds the practically steady value which would 

 exist round J if S were absent. 



Consequently the concentration of the mercury salt round 

 S must tend to diminish continuously until it is as small as 

 that round J. But if, before this can happen, the amount of 

 Hg in solution at S becomes smaller than that required to 

 satisfy (i. a), a new reaction will begin. Direct action 

 between the mercury of S and the acid will occur with 

 evolution of hydrogen and formation of a new supply of 

 mercury salt in solution. 



This action will be continuous, for, in virtue of the con- 

 tinuous effect of J, Hg will be continuously removed at S. 

 A steady state (neglecting secondary actions such as described 

 below, § 12) will be reached when the rate of evolution of 



