Chemical Affinity in terms of Electromotive Force. 191 



In these three cases the maximum B.M.F. was attained 

 respectively after about 3-7, 7-10, and 7 minutes. The value 

 of the maximum was much higher in the second case than in 

 the first, indicating a much greater amount of gas occluded, 

 as might be anticipated, since a much more rapid current 

 flowed during the period before the electrodes were discon- 

 nected from the battery, the current being near to '000001 

 and *000020 r weber during these periods in the first and 

 second cases respectively* Notwithstanding, however, that a 

 still more rapid current (about '000025) flowed in the third 

 case, the maximum attained was scarcely higher than that in 

 the second, because the smaller surface of the electrode caused 

 the rate of loss of aura by solution in the unsaturated sur- 

 rounding fluid and diffusion away of the solution thus formed 

 to another part of the U-tube to be relatively larger in this 

 case ; consequently the rate of fall after the maximum had 

 been reached was considerably more rapid. 



73. A curious effect is sometimes produced by the gradual 

 passage outwards from electrodes of occluded gases and the 

 formation of an aura round each, thus virtually setting up a 

 kind of Grove's gas battery. If the current passing through 

 a voltameter be suddenly largely diminished (but not altogether 

 interrupted) by diminishing the E.M.F. of the battery used, 

 a considerable decrease is brought about in the counter 

 E.M.F. set up in the voltameter ; that is, the value of 

 e = E!+ [2{(l-w)H}-S(nfc)-2(H)] X J diminishes. The 

 passage outwards of occluded gas from the interior of the elec- 

 trodes, however, tends to increase the aura round each electrode, 

 and consequently to diminish 2(w/i), and hence to increase e; 

 and the result is that the voltameter behaves for a while like a 

 more powerful opposed battery; so that the current for a time 

 passes in the opposite direction to that due to the primary elec- 

 tromotor : of course this effect can only be produced svhen the 

 E.M.F. of the primary electromotor (after the reduction in its 

 E.M.F.) does not exceed some particular limit. If the expe- 

 riment be modified by shortcircuiting the electrodes of the 

 voltameter before again coupling them to the battery after 

 reducing its E.M.F., the current will at first pass in the normal 

 direction clue to the battery ; but as the occluded gases pass 

 outwards from the interior of the electrodes and so form an 

 aura round each, the current passing gradually diminishes in 

 strength until it becomes nil, and finally passes in the oppo- 

 site direction, just as it would have done at first had the vol- 

 tameter not been shortcircuited. For example, a current of 

 about '00005 weber (capable of evolving about 0*5 cubic cen- 

 tim. of hvclrogen per day) was passed for several days through 



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