Electrodes in Water free from Air. 143 



that the polarization observed was in some way or other due to 

 the presence of dissolved air, and that in air-free liquid the 

 effect would not have been observed, at any rate with the elec- 

 tromotive force (about g-J-Q of a Daniell's cell) which came 

 into play. 



To meet these doubts two very definite questions have to be 

 examined. 



(i) In air-free water, can platinum electrodes be polarized 

 by very small electromotive forces to the same degree and with 

 the same facility as in ordinary aerated water ? 



(ii) Is this very feeble polarization really a decomposition 

 of the electrolyte ? 



If the following facts are held to give an affirmative answer 

 to both the above questions, the objections raised are invalid 

 and may be dismissed. 



To test the first point, it was necessary to compare the effect 

 of a small known electromotive force acting through platinum 

 electrodes, first' in air free, next in aerated acidulated water, 

 under identical conditions. In the former experiments the 

 electromotive force was generated in the liquid by its motion ; 

 for my present purpose it was sufficient to act on it by an ex- 

 ternal electromotive force of measurable amount, since it is 

 obvious the results hold good no matter how it is produced. 

 The arrangement consisted of the following parts. 



The decomposing cell or voltameter was a glass tube 30 cen- 

 tims. long, 2 J centims. wide, stopped at one end by an india- 

 rubber stopper, through which passed two platinum wires 

 having welded to them on the interior platinum plates 8 cen- 

 tims. long, 2 centims. wide, placed so as to lie against the walls 

 of the tube. The other end of the tube was connected by a 

 narrow tube with the Sprengel pump. The platinum plates 

 having been rendered chemically clean, the tube was three 

 parts filled with dilute sulphuric acid, which had been previ- 

 ously boiled for six hours so as to expel some of the dissolved 

 air. A vacuum was then made in the voltameter ; and the ap- 

 paratus remained untouched for five days, the pumping being 

 carried on at intervals and continued during the experiments. 



To obtain a small electromotive force a well-known method 

 was made use of. A Wheatstone's bridge had its branches 

 filled up with previously measured resistances, and the other con- 

 nexions made as in the figure (p. 144). Between the points P 

 and Q any small difference of potentials can be produced whose 

 value is known when the distance x Q of the block from the 

 point of equilibrium x is known. The galvanometer employed 

 was a very sensitive mirror-galvanometer, with short needle 

 and a long suspending fibre. The image of a fine wire across 



