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



time the only effect of J at S is to produce sulphur. If, 

 however, the capillary at S is rinsed out by expelling a little 

 of the mercury, the evolution of hydrogen begins again at 

 the fresh surface of acid and mercury. 



(h) When the diameter of S is large — for example, twenty 

 times that of J — sulphur and sulphide only are formed, how- 

 ever efficient the jet may be. In this case the jet only slowly 

 reduces the amount of mercury salt in solution round S, and 

 the reactions depending on the formation of sulphuretted 

 hydrogen begin before the evolution of hydrogen is possible 

 and continue in such a way that it can never occur. In a 

 qualitative sense it may be said that in this case the slower 

 reaction (requiring greater molecular rearrangements) has 

 time to prevent the first. 



It is only when the acid is concentrated that these effects 

 occur, for diluted sulphuric acid is not reduced to or by 

 sulphuretted hydrogen. Thus even in the case (b) just men- 

 tioned, hydrogen is evolved freely when sulphuric acid 

 solution of s. G. 1*5 is substituted for the concentrated. 



§ 13. Effects of Dilution of the Acids. — The loss of available 

 energy when an equivalent of hydrogen leaves the acid 

 solution and is evolved as gas at atmospheric pressure falls 

 continuously* as the solution is diluted. Consequently the 

 amount of mercury which can be in solution round the 

 electrode without preventing the evolution of hydrogen 

 becomes continuously less. 



Before the hydrogen can escape it must reach a certain 

 concentration (in the neutral state) in the mercury and in the 

 solution at the surface layer. Disregarding the difficulty of 

 formation of very minute bubbles, evolution just fails to occur 

 when, electrode and solution being saturated, there would be 

 no loss of available energy if an infinitesimally small quantity 

 of hydrogen left the solution and the infinitesimal equivalent 

 of mercury entered. 



Thus if* there is a limit below which J (however effective) 

 cannot reduce the concentration of the mercury salt round S 

 (§9), there is also a limit to the concentration of the acid 

 which can be decomposed at S with evolution of gas. The 



* In the case of concentrated sulphuric acid, dilution seems first to 

 increase the rate of evolution of hydrogen. This result might be 

 anticipated for two reasons. The first effect of dilution is to increase 

 the conductivity and, probably, the ionic concentration of the acid. 



Thus the effectiveness of the jet and the value of v k will simultaneously 



rise. Again, some of the hydrogen which would otherwise escape 

 will react with the concentrated acid while in the nascent state (§ 12). 

 There should thus be a particular strength of sulphuric acid for which 

 the rate of evolution of gas is a maximum. 



