196 ELECTRIC FISHES. L.ECT. X. 



of iodide of potassium ; and, lastly, close the circuit by ef- 

 fecting a communication between the discs by means of a 

 platinum wire, we soon find that at each discharge of the 

 fish, a reddish yellow spot is formed around the extremity 

 of the platinum wire, which touches the piece of paper 

 placed on the platinum of the ventral surface. The paper 

 on the platinum disc in contact with the back of the 

 animal, is coloured also, but more feebly. The solution 

 which impregnates the paper is, therefore, decomposed by 

 the current of the torpedo, and iodine is evolved at the po- 

 sitive pole. 



Fig. 13. 



Torpedo placed between the Sole and Cover of an Electrophorus. 



We may also succeed in observing the spark at the mo- 

 ment of the discharge. The apparatus employed to effect 

 this is very simple : place a very lively torpedo upon a 

 large metallic plate, like that of a perfectly insulated elec- 

 trophorus, and then put above the fish a disc having an in- 

 sulating handle. Each of the two parts of the apparatus 

 should be furnished with a metallic wire or rod, to the 

 upper extremity of which is attached, by means of gum, a 

 piece of gold leaf; the two leaves, consequently, hang 

 downwards. We arrange the plates so as to bring the two 

 leaves near to each other. By compressing the fish by 

 means of the upper plate, and by bringing the two gold 

 leaves almost in contact, we frequently see the spark pass 



