1888.] Development of Electricity by Atmospheric Oxidation. 197 



Volts. Observed values of K 2 . 



Copper replaced by zinc = +0*960 + 0-920 to + 0'960 

 Copper replaced by silver = — OvoO — 0-340 to — 385 



Whilst with zinc, lead, copper, and cadminm, the observed values of 

 K 2 in no case differ very widely from those equivalent to the differ- 

 ences in heat of formation, those observed with silver show large 

 differences, indicating as before that silver exhibits a high negative 

 value for its thermo-voltaic constant in each case, viz., — 0'5 to — 0'6 ■ 

 in contact with dilute sulphuric acid, and near to — 0"4 in contact 

 with ammoniacal fluids, this latter value being close to those found • 

 previously for silver in contact with neutral solutions of its sulphate, 

 nitrate, and acetate (loc. cii.). 



On the whole, except when an oxidisable metal is used exhibiting 

 a high negative value for its thermo-voltaic constant, the E.M.F. of 

 a cell containing an aeration plate and an oxidisable metal always 

 falls short, and sometimes largely short, of that equivalent ' to the 

 chemical changes going on therein even under the most favourable 

 conditions when generating only an infinitesimal current, the de- 

 ficiency being still more marked when the current density is not so 

 minute: in other words, the modus operandi of cells of this class is 

 such as necessarily to render a large fraction of the energy non- 

 adjuvant so far as current is concerned. Just the same remarks 

 apply, as far as our experiments have gone, to cells in which the 

 oxidisable substance is in solution, an extreme case of which is 

 exhibited by cells set up with a solation of sulphurous acid and a sub- 

 merged platinum foil plate, opposed to an aeration plate of platinum 

 sponge on the surface of dilute sulphuric acid. Such cells give an 

 E.M.F. (when generating only extremely small currents) of from 02 

 to 0*3 volt, whilst the heat of oxidation of sulphurous acid solution, 

 S0 2 aq,0, is 63634 gram-degrees, according to Julius Thomsen. 

 corresponding with 1368 volt, or upwards of a volt more than that 

 actually produced.* Analogous diminutions in E.M.F. are brought 

 about in many other cases, to extents depending not only on the 

 nature of the aeration plate but also on that of the oxidisable fluid. 



* A large part of the depreciation in this case is due to the fact that sulphurous 

 acid solution and platinum constitute an oxidisable portion of a cell behaving as 

 magnesium and aliuninium do in cells where they replace zinc, i.e., giving a much 

 smaller E.M.F. than that due to the heat corresponding with the chemical change : 

 thus, if a cell be set up with zinc or dilute sulphuric acid opposed to platinum in 

 sulphuric chromic acid solution, and the zinc and sulphuric acid be then replaced 

 by platinum and sulphurous acid solution, the E.M.F. falls by an amount 

 greater by 045 to 5 volt than that corresponding with the differences in heat evolu- 

 tion between Zn,0,SO;,aq anc l SOoaq.O (viz., 106090 — 63634 = 42456 gram-degrees 



= 913 volt) : and similarly with other oxidising fluids. Solutions of alkaline 



sulphites behave similarly. 



p 2 



