Action between Metals and Acids. 843 



sulphide of mercury by interaction with the sulphate in 

 solution. The former reaction may be represented by the 

 equation 



3Hg 2 + 4H 2 S0 4 = 3Hg 2 S0 4 + S + 4H 2 0. . (II. a) 



In this case, neglecting for simplicity the energy variation 

 due to the decomposition of the S0 4 ion, the equilibrium 



value of jjb ni may be represented qualitatively by 



s 4c I s H. 2 0\ . n ,... . 



^» = 3r* -^" ) +■«/ + /*», • • • On.) 



which shows that more mercury must now enter solution 

 before equilibrium is attained than when hydrogen ceases to 

 be evolved in accordance with (i.a). The reaction with 

 precipitation of mercury sulphide leads to a similar result. 



Thus we are led to infer that the production of sulphur 

 and of sulphide of mercury may continue after the evolution 

 of hydrogen has ceased. 



This inference is fully confirmed by experiment : — 



(a) When the diameter of S is small — less or not much 

 greater than that of J — the mercury in solution round S is 

 removed almost at once by the action of J and hydrogen 

 simultaneously appears. Very soon, however, a yellowish- 

 white cloud begins to form (particularly round the portions 

 of S where the curvature is least and where the evolution of 

 hydrogen is most noticeable). This cloud probably arises 

 mainly from the decomposition of H 2 S. In fact when the 

 effectiveness of J was decreased by reducing the head some 

 of the last bubbles to escape seemed (when observed through 

 a microscope) to be surrounded by a film exactly like that 

 which is produced when bubbles of H 2 S are passed into 

 concentrated H 2 S0 4 . The formation of sulphide of mercury 

 can also be detected after the jet has been in action for 

 some time. 



These reactions rapidly reduce the rate of evolution of 

 hydrogen at S. The production of sulphur is a process which 

 the action of J cannot reverse and the amount increases 

 continuouslv, raising the electric resistance of the solution in 

 the capillary and hence diminishing the effectiveness of J. 

 The insoluble sulphide precipitated on the surface of the 

 mercury further increases the circuit resistance. In addition 

 this sulphide seems to prevent the evolution of hydrogen 

 directly although the amount of mercury salt in solution may 

 be very small. For instead of escaping, the hydrogen must 

 now apparently interact with the sulphide in the surface 

 layer producing ELS and eventually sulphur. Thus after a 



