Chemical Affinity in terms of Electromotive Force. 265 



of hydrogen is due to that cause of loss of hydrogen which 

 cannot possibly be entirely removed, viz. the diffusion to the 

 hydrogen plate of oxygen-containing liquid, and vice versa; 

 whilst the larger deficiencies at first noticed are due to this 

 cause increased by the solution of hydrogen in the fluid, its 

 occlusion by the electrode, and its oxidation by the last traces 

 of unreinoved air. That this is the true cause of the residual 

 constant deficiency is moreover further indicated by the fact 

 that the nearer together the electrodes and the wider the 

 bore of the U-tube, AB (that is, the more rapid the rate of dif- 

 fusion), the greater, cceteris paribus, is the residual deficiency; 

 whilst fluctuations of temperature, which would necessarily 

 aid the diffusion of the gaseous solutions by setting up con- 

 vection currents, considerably increase the deficiency. It 

 might be supposed that perceptible errors in measurement of 

 the hydrogen would be occasioned by the adherence to the 

 electrode K of visible minute bubbles of gas ; in practice, 

 however, it was always found that gently tapping the U-tube 

 AB caused such bubbles to rise so completely into the bell D, 

 that no visible bubbles still adherent to K could be distin- 

 guished. 



80. In order to determine quantitatively the amount of 

 loss of hvdrogen due to this unavoidable " diffusion discharge " 

 (as for want of a better name it may be termed), a large 

 number of observations were made with the different volta- 

 meters employed in the following way. Firstly, a current 

 of 25 to 50 microwebers was passed until the residual deficiency 

 became constant (for a given current); and then the current 

 was gradually diminished at stages of two or three days 

 apart, determinations of the hydrogen actually evolved per 

 day, or per two or three days, being made. By-and-by a 

 particular limiting current-value was arrived at, such that 

 with the particular voltameter used no gas whatever was 

 collected, no matter how long the current was allowed to pass, 

 if the current-strength were below that limiting value ; whilst 

 if the current were above that amount, and provided the tempe- 

 rature was as nearly uniform as circumstances permitted, so that 

 this effect of heat-convection currents was reduced to a mini- 

 mum, a quantity of hydrogen was collected after a given time, 

 the greater the larger the current-value. In this way the 

 following limiting values were obtained with four voltameters, 

 some of the particulars of the construction of which are 

 annexed : — 



Voltameter No. 1. Electrodes of platinum-foil bent into 

 cylinders so as to be about concentric with the walls of the 

 U-tube. Total surface of each electrode (reckoning both sides 



