Chemical Affinity in terms of Electromotive Force. 175 



making its appearance as a free gas by using a large electrode 

 and a minute one (Wollaston's point), a feeble evolution of 

 gas may be noticed (especially at first) from the small elec- 

 trode, but none from the other. Similarly, the experiments 

 of Helmholtz on what he terms "electrolytic convection" 

 (Phil. Mag. [4] xlvii. p. 152), and analogous observations of 

 others, show that, if the one or the other gas is suppressed by 

 the chemical action of other kinds of gas respectively dissolved 

 in the fluid surrounding the electrode or adherent to the elec- 

 trode surface, water may be decomposed by an E.M.F. of less 

 than 1*50 volt. 



59. As regards " subsequent polarization," or the E.M.F. 

 existent between the electrodes of an electrolytic cell after 

 the original current has been ruptured, even when measured 

 by a quick- working switch or " wippe," it results from the 

 observations at present on record that a more or less percep- 

 tibly smaller value is always obtained than corresponds to 

 the counter E.M.F. set up whilst the current was passing. 

 Under certain circumstances, especially when currents of 

 short duration are employed, the difference becomes very 

 marked ; so that on the whole " subsequent polarizations " 

 have been recorded, for acidulated water for instance, of 

 magnitude varying from something upwards of two or even 

 three volts to something less than '001 volt. With currents 

 producing smaller values than 1*5 volt (measured immediately 

 after rupture) visible evolution of gas at both poles does not 

 seem to have ever been noticed. Recently Exner (Wied. 

 Annalen, vi. p. 336, 1870) has shown that to cause visible evo- 

 lution of gas from boiled-out distilled water, a " subsequent 

 polarization " (measured immediately after rupture by a 

 quadrant electrometer) of from 2*03 to 2*09 Daniell cells (from 

 2*2 to 2*3 volts) must be established ; whilst if the water be 

 faintly acidulated, a polarization of about 1*89 Daniell =2*1 

 volts is sufficient. And in the same paper he has also shown 

 that the polarization set up after the current is ruptured 

 differs from the E.M.F. of the battery employed (and a fortiori 

 from the counter E.M.F. set up during the passage of the 

 current) only by quantities too small to measure accurately by 

 a quadrant electrometer, when the battery E.M.F. does not 

 exceed from l'40to 1*47 Daniell cell (1*54 to 1*61 volt) when 

 larger platinum plates and boiled-out dilute acid are used, and 

 from 1-94 to 2'09 Daniell cells (2*1 to 2'3 volts) with Wollas- 

 ton's points (platinum wires coated with glass, so as only to 

 expose a minute surface at the ends) ; whilst with stronger 

 battery electromotive forces than these values the subsequent 

 polarization always falls sensibly short of the battery E.M.F. 



02 



