188 Dr. C. 11. A. Wright on the Determination of 



platinum, or carbon, whatever their size, and whatever the 

 value of E, in all cases the general character of the numbers 

 was the same as in these two examples ; i. e. the rate of fall 

 with acid containing dissolved air was greater than that with 

 acid freed from the greater part of the air dissolved by boiling ; 

 whilst this again was greater than that with acid from which 

 the last traces of dissolved air had been removed (from the fluid 

 surrounding the hydrogen-pole) by the slow evolution of hy- 

 drogen therefrom, so as to saturate the fluid with hydrogen 

 and remove the dissolved oxygen by " electrolytic convection." 

 As the removal of the last traces of oxygen became more and 

 more nearly effected, the rate of fall of polarization (for a given 

 initial value) gradually approached a minimum value, beyond 

 which it never sank, this minimum doubtless representing the 

 rate of alteration produced in the auras by diffusion from one 

 electrode to the other. 



71. The following experiments are specimens of a number of 

 other analogous ones, all of which yielded the same general 

 result, viz. that, other things being equal, the rate of fall of 

 polarization is less the larger the electrode surface. The volta- 

 meters were precisely like those used in the above experiments, 

 consisting of U-tubes with pieces of platinum-foil bent into 

 cylinders and attached to platinum wires fused into glass tubes 

 which served as mercury-cups — access of external air being- 

 prevented by closing each of the two open ends of the U-tubes 

 with a doubly perforated cork (of india rubber, or paraffin), 

 the mercury-cup tube passing air-tight through one perforation 

 and a delivery -tube (bent over and dipping under mercury or 

 oil) through the other, so that evolved gas could escape with- 

 out admission of air. The voltameters, being filled with boiled 

 acid containing about 27 grammes of H 2 S0 4 per 100 cubic 

 centims., were then connected with a pair of Minotti cells, a 

 large resistance being in circuit; so that a feeble current passed, 

 evolving one or two cubic centims. of hydrogen per day. 

 After some days (generally a week to a fortnight) the mini- 

 mum rate of fall of polarization was arrived at ; when this was 

 the case, the electrodes were kept for another day at some par- 

 ticular difference of potential, arrived at by suitably altering 

 the current passing and experimented with throughout, and a 

 series of observations made ; the electrodes were then again 

 brought to the same difference of potential by passing the 

 current again for some time, and another series of observations 

 made ; and so on. Finally, the following values were obtained 

 from several such series of concordant observations, the expe- 

 riments being alike in every respect save size of electrodes — 

 the strength of the acid and the distance apart of the nearest 

 portions of the electrodes being as nearly as possible the same 



