390 Mr. J. Brown on Helmholtz's Theory of 



question which he attempts to answer by a comparison of the 

 indication of the dropping electrode M'ith that of a capillary 

 electrometer, and the adoption of several hypotheses which 

 cannot be said to appeal directly to one's sense of exactness. 



17. In the paper referred to above by Exner and Tuma 

 these authors refer to the improbability of any considerable 

 current being produced by the mere carrying down of electric 

 charges on the falling mercury drops : and they adduce ex- 

 perimental evidence to show that such charges are not pro- 

 duced by any simple contact action between the mercury and 

 electrolyte, as supposed by Ostwald, who seems to regard the 

 arrangement as annlogous to the water-dropping collector of 

 Sir W. Thomson. The conditions, however, are quite different 

 from those of Sir W. Thomson^s collector, where the potential 

 measured is that originally existing at the point where the 

 drops form, and is usually independent of any action between 

 the drops and the medium in w'hich they form. 



18. Their paper then puts forward as the true theory of the 

 observed current the explanation given in § 9, viz. that the 

 arrangement is simply a galvanic cell : clean mercury — acid — 

 oxidized mei'cury ; and it is further pointed out that the 

 current carries by electrolytic convection hydrogen to the 

 resting mercury whereby the latter is deoxidized. 



It would appear therefore that, leaving local action out of 

 account, the oxidizing of the dropping, and deoxidizing of 

 the resting, mercury go on simultaneously ; and as the oxide 

 formed on the drops falls on the resting mercury, I presume 

 we must conclude that the ultimate source of current in this 

 cell is the gravity potential of the upper mercury, which is 

 used up in producing the drops, and changes first into energy 

 of surface tension and then into current. 



19. In conclusion, it is important to remark that the view 

 here adopted implies that electrolytes, such as dilute acids, 

 which attack mercury, are positive to clean mercury in contact 

 with them ; for if not, and if the electrolyte is negative to the 

 dropping mercury, it must (in order that the observed current 

 may be produced) be still more negative to the resting 

 mercury ; that is to say, the difference of electric potential 

 is least where the conversion of chemical potential energy is 

 greatest — a rather improbable assumption, and one not borne 

 out by analogies with other voltaic combinations. 



If this be admitted, it follows that the results of all those 

 recently made researches which are based on the assumption 

 that mercury is positive to electrolytes, must be considered 

 doubtful. 



