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LIV. On the Theory of Dropping Electrodes. — Reply to 

 Mr. Brown. By Prof. W. Ostwald*. 



IN the April number of the Philosophical Magazine, Mr. 

 Brown raises objection to a remark, which I made in the 

 Zeitschr. f. ph. Ch. iv. pp. 577, 578, about his supposed dis- 

 proof of v. Helmholtz's Theory of Dropping Electrodes. I 

 must regret not having expressed myself clearly enough in 

 that remark, as Mr. Brown has not considered its essential 

 part at all in his note. 



Mr. Brown ascribes to v. Helmholtz the opinion that the 

 cause of the galvanic current observed when a mass of mer- 

 cury dropping in an electrolyte is connected by means of a 

 galvanometer with the mercury collecting at the bottom of 

 the vessel, is the " charges carried down by the drops." I do 

 not know on what passage of Helmholtz's writings Mr. Brown 

 supports this statement ; any such is unknown to me. Helm- 

 holtz's assumption is, that every drop on its formation becomes 

 covered with an electrical double layer similar to the charge of 

 a Leyden jar. This double layer contains positive and nega- 

 tive electricity in equal quantities ; its motion cannot therefore 

 give rise to anything that is similar to a. galvanic current. 

 From this it follows, conversely, that the galvanic currents 

 produced in this way have no connexion whatever with the 

 motions of the drops. 



Such a connexion, however, exists with the formation and 

 the destruction of the drops. Every drop that is produced 

 takes positive electricity from the mercury, and negative from 

 the electrolyte, in order to form its double layer. On the 

 union of the drop with the mass of mercury lying at the 

 bottom, the positive electricity goes over to the latter while 

 the negative electricity remains in the electrolyte, and moves, 

 in case the process continues, to the point of formation of the 

 drops, to produce there new double layers. This latter move- 

 ment is of course dependent on the resistance of the electrolyte, 

 and Mr. Brown's observation therefore is not, as he believes, 

 in contradiction, but in complete agreement with the theory 

 of Helmholtz. 



Against this, Mr. Brown objects that currents are also ob- 

 tained if the dropping mercury is connected with a secondary 

 mercury electrode which receives no drops. It seemed to me 

 unnecessary to develop the above-given explanation also for 

 this case, since every one who understands that can also here 

 apply it. On Mr. Brown's account I will, however, again 



, «,£« 



Communicated by the Author. 



