270 Dr. C. R. A. Wright and Mr. C. Thompson, pec. 15, 



ment not reckoning inequalities of sponge) were used, and in No. 2 

 plates only about one-fifth that size. 



It is obvious that during the passage of a current the dilute 

 sulphuric acid between the two plates must be electrolysed, so that 

 hydrogen would tend to be liberated on the surface of the plate 

 acquiring the higher potential, and oxygen on that of the other; the 

 hydrogen whilst nascent would necessarily be more or less completely 

 oxidised to water by the oxygen of the film of condensed air, so that 

 on the whole the net chemical action in the cell itself would be 

 either nil (if all hydrogen were so re-oxidised) or one absorbing heat 

 (if some of the hydrogen escaped oxidation). The oxygen slowly 

 evolved would escape as such, being dissolved by the surrounding 

 fluid. The effect of this should accordingly be that the efficiency of 

 the air film on the first plate would be more or less depreciated, and 

 that on the second exalted ; in point of fact, if the two aeration plates 

 in such an arrangement, which has been generating a current for 

 some time, be (by means of an appropriate switch) disconnected 

 from one another and successively opposed to a given oxidisable plate, 

 the one does give a considerably lower and the other usually an 

 appreciably higher value than the constant ones previously obtained 

 ( >efore the two aeration plates were directly opposed to one another) 

 o i opposing each severally to the oxidisable metal ; whilst on allowing 

 the cell to stand for some time generating no current, the lower 

 value gradually rises and the raised one falls until sensibly the old 

 constant values are again obtained. 



We noticed, moreover, that when aeration plates of platinum-foil 

 or sponge are used opposed to silver plates in conjunction with a fluid 

 capable of dissolving silver oxide (such as dilute sulphuric or acetic 

 acid or ammonia solution) distinctly larger amounts of current are 

 usually developed than when opposed to carbon or gold plates, 

 and that simultaneously silver passes into solution, the silver plate 

 acquiring the lower potential, diminishing in weight, and, in short, 

 behaving precisely as though it were an oxidisable metal, such as zinc 

 or copper. Obviously this is due to the circumstance that with silver 

 the oxygen liberated attacks the metal of the plate acquiring the lower 

 potential ; but the remarkable part of the action is that this attack is 

 only partial, so that the amount of silver dissolved is invariably less than 

 that equivalent to the current passing, i.e., less than that deposited in a 

 silver voltameter included in the circuit. Thus the following numbers 

 were obtained in a series of experiments, in each of which four 

 similar cells containing platinum sponge aeration plates arranged in 

 series were used in order to shorten the time of observation. The 

 electrolytic fluids used in the various cases were respectively : — 



