Chemical Affinity in terms of Electromotive Force. 367 



its current whilst a gramme-equivalent of copper was precipi- 

 tated. The difference between the heat actually evolved in 

 the cell (measured by a calorimeter) and that equivalent to 

 the polarization, was termed by Raoult " Local Heat," and 

 found by him to be larger with stronger currents. The 

 results above described with a Daniell cell, however, show 

 that the valuation in terms of heat of a Daniell cell is not a 

 constant quantity ; for the heat capable of being evolved 

 by the current outside the battery during the precipitation 

 of a gramme-equivalent of copper, being proportional to the 

 E.M.F. of the cell, is variable with the time during which this 

 precipitation takes place, i. e. with the strength of the current ; 

 whilst the " subsequent polarization," measured by Raoult, 

 being always more or less inferior to the counter E.M.F. set 

 up during the passage of the current, is not the true mea- 

 sure of the electrolytic work done by the current whilst it was 

 passing through the cell.] 



Notwithstanding, however, that the numerical results of 

 Favre and Raoult are open to exception, these observers 

 were amongst the first, if not actually the first, to attempt to 

 carry out experimentally some of the lines of research opened 

 out by Sir W. Thomson (1851 paper), although the theo- 

 retical aspects of the question, as discussed by them, do not 

 entirely coincide with Thomson's mode of reasoning. 



Summary of Results. 

 101. The following summary briefly indicates the salient 

 results arrived at in Parts III. and IV. of these researches. 



(1) The passage of a given quantity of electricity through 

 an electrolyte causes the decomposition of one and the same 

 amount of substance, irrespective of the time taken in its 

 passage ; in other words, conduction without electrolysis does 

 not take place, and Faraday's law is true for excessively 

 minute currents as well as for those of considerable magnitude. 



(2) With very feeble currents, however, and with certain 

 electrolytes, e. g, water, the quantity of products of decompo- 

 sition actually collected after a given time, does not absolutely 

 correspond to the quantity of electricity that has passed, even 

 after various sources of suppression have been eliminated, such 

 as occlusion in or condensation on the electrodes, solution in 

 the fluid, or suppression by the chemical action of dissolved 

 gases. The cause of this is the " diffusion discharge " 

 produced (in the case of water) by the diffusion towards the 

 — electrode of water containing dissolved oxygen, and to- 

 wards the + electrode of fluid containing dissolved hydrogen 

 (and similarly in other cases), thus causing an unavoidable 



