1888.] Development of Electricity by Atmospheric Oxidation. 183 



ferric sulphate or sulphuric acid respectively. A considerable 

 enhancement in the EJVI.F. of such an arrangement is effected if the 

 oxidisable fluid is rendered alkaline, and the aeration plate surrounded 

 by acid fluid, as in that case the passage of a current is also accom- 

 panied by the mutual neutralisation of the acid and alkali to an 

 extent equivalent to the current flowing (apart from diffusion) : the 

 heat of neutralisation of caustic soda and sulphuric acid is (Julius 

 Thomsen) 31,378 gram-degrees, equivalent to 0675 volt ; and 

 similarly in other cases. Thus, tolerably energetic aeration cells are 

 obtained by using caustic soda solution of pyrogallol or sodium 

 hyposulphite (Schutzenberger's hydro sulphite), and a platinum sponge 

 aeration plate on the surface of diluted sulphuric acid ; and somewhat 

 weaker ones if an alkaline solution of ferro-cyanide or sodium plumbate 

 be used, ferri-cyanide or lead dioxide (precipitated in the solid form 

 on the electrode) being the product of the oxidation thus effected, 

 and the E.M.F. beiug upwards of 0*8 volt in each case. 



In all aeration cells, whether oxidisable fluids or metals be employed, 

 one marked feature is the extremely rapid rate at which the E.M.F. 

 of the cell falls if the current generated is made to exceed a very 

 small limiting density relatively to the area of the aeration plate. 

 Of course when this plate is a sheet of polished metal such as platinum 

 foil, this limit lies much lower than when it is a tray of the same 

 area filled with spongy metal, pulverised graphite, &c, because in the 

 latter case the true surface acting is much greater than the actual 

 area of the tray : a number of observations led us to the conclusion 

 that with the most sensitive kinds of aeration plates examined (thin 

 foils or leaves of the precious metals), a measurable depreciation in the 

 E.M.F. of a cell that otherwise would give a constant value, was 

 speedily brought about if made to generate a current of greater 

 density relatively to the aeration plate than about micro-ampere 

 per square centimetre of surface, or 1 micro-ampere for a plate 

 5 centimetres square, exposing 25 square centimetres of surface (one 

 side only reckoned) ; but with aeration plates of spongy metal 

 currents of many times this density produced little or no depreciation 

 even after flowing some time. Even with the most favourable kinds 

 of plates, however, the tendency towards depreciation was so far 

 marked as to render it evident that but little hope could be enter- 

 tained of utilising the principle of atmospheric oxidation for the 

 production of cheap currents of sufficient power for practical use, 

 e xcepting when plates of enormous area are employed, so that the 

 density of the current should still be small, even when the current 

 itself was of moderate magnitude. Admitting, however, that a large 

 superficial area (e.g., a lake or artificial reservoir) of fluid were avail- 

 able, and that the cost of a proportionately large system of aeration 

 plates were not prohibitive, iu does not seem absolutely impossible 



