420 ELECTRO-PHYSIOLOGY CHAP. 



Electrolysis of potassium iodide was often used instead of 

 the multiplier to determine the direction of shock in the fish and 

 the distribution of surface-tension. The discharge was then led in 

 by two annealed platinum points to a strip of filter-paper saturated 

 with solution of potassium iodide. Du Bois-Eeymond (4 c, p. 163 

 and 7) then encountered the paradoxical phenomenon that in the 

 discharge from both Malapterurus and, as he found later, Torpedo, 

 an iodine spot appeared under loth electrodes, but was as a rule 

 more distinct in the former beneath the point corresponding with 

 the tail. John Davy and Matteucci had not remarked this effect 

 on Torpedo, nor Faraday, Schonlein, and others on Gymnotus, 

 and Sachs also failed to obtain it in the latter. 



Since the alternation of the discharge thus seemed possible, 

 a closer examination was required. It then appeared that the 

 " secondary " iodine spot under the negative electrode can be 

 produced by single induction -shocks also, if, as is usually the 

 case, the circuit is left closed after the current has ceased to flow. 

 Here it is undoubtedly due to the " current from the opposite 

 discharges received by the platinum points dipping into the 

 iodide of potassium solution, under the action of the induction 

 current." " The process in the fish is quite similar to that in 

 the induction circuit. The circuit remains closed for some 

 moments after the shock has been given, however quickly the 

 saddle be lifted out of the water, there being, moreover, no especial 

 reason for haste. During this time a secondary current must 

 cross the current of the fish in the opposite direction. This is 

 derived not merely from the charges of the platinum points 

 which dip into the iodide of potassium solution, but from those 

 of the platinum saddle also. This secondary current must in- 

 evitably produce a corresponding spot of iodine under the previous 

 kathode and present anode" (4 d, p. 651 f.). Du Bois-Eeymond 

 proved by experiment that demonstrable polarisation of the 

 electrodes does occur from the discharge of the fish. The current 

 from the shock was conveniently kept away from the galvanometer 

 (by the frog -interrupter) through a derivation circuit, and in 

 order to make the polarisation visible, it was only necessary to 

 open this as soon as possible after the shock. 



The electrical manifestations in those species which were 

 termed above " pseudo-electric " (Raja, Mormyrus) are much less 

 conspicuous than in the electrical fishes proper, as described 



