DR. DAVY ON THE TORPEDO. 545 



When besides the human body and the chemical apparatus the galvanometer has 

 been introduced into the circle with the air thermometer and spiral, the shock has 

 been experienced as if it had been received direct from the fish, but I have never 

 witnessed at the same time any other effect. 



Not taking the human body into the circle in trials on fish of veiy feeble elec- 

 tricity, I have witnessed the precipitation of iodine when neither the air thermometer, 

 nor a delicate galvanometer with a double needle, has been affected. 



The same kind of evidence has been obtained of the thermometrical test being 

 next in point of delicacy, in as much as I have seen the air thermometer affected by a 

 fish which had no influence on the galvanometer in connexion with the wire of the 

 thermometer. 



That the needle in the spiral is the least delicate test, does not require to be in- 

 sisted on. The electricity of a Torpedo, of almost feeble energy, has been equal to 

 produce all the effects alluded to at once, excepting sensation, as already explained, 

 and excepting the imparting permanent magnetism to the steel needle. The last 

 effect, as might be expected, has required greater force ; a moderate force, however, 

 has been sufficient when a very slender needle has been used, and a spiral of fine 

 wire closely coiled, only just capable of receiving the needle*. 



It having been stated on high authority that a spark has been obtained from the 

 Gymnotus electricus\, I have thought it right to renew the attempt to procure a 

 spark from the Torpedo. I have tried the method which it is said succeeded with 

 Mr. Walsh in the instance of the Gymnotus, namely, dividing with a pen-knife gold- 

 leaf attached to glass, and connecting the divided parts with the contact wires. 

 Using an active fish in this way I could neither observe a spark in the dark, nor in 

 the light detect the slightest indications of the passage of electricity, either by 

 the galvanometer or the more delicate test of the sensation or shock. I have been 

 equally unsuccessful using an electroscope formed on the principle of Coulomb's, 

 which displayed sparks when touched either by a small rod of glass slightly excited, 

 or of sealing-wax ; even when the Torpedo was taken out of water and all adhering 

 moisture removed, no effect could be obtained, not even the slightest indications of 

 attraction. I have varied the trials, using highly rarefied air at ordinary tempera- 

 tures, and also condensed air deprived of moisture, with the same negative result. 

 And I have been equally unsuccessful in substituting flame : unless the metallic 

 points were in contact in the flame of the spirit-lamp, the passage of the electricity 



* The spiral I have latterly used is of fine copper wire gUt, having about 300 convolutions to the inch : an 

 inch of it weighs four tenths of a grain. 



t Mr. Walsh is said to have written to M. le Roi to the above effect ; and also that Sir John Pringle 

 and M. Magellan assured M. le Roi they had witnessed the result repeatedly. Vide Bloch's Ichthyologie, 

 p. 1020. Bloch refers for his information to Rozier's Journal, Ann. 1774. M. de Humboldt (Annales de 

 Chimie et de Physique, torn. xi. 427.) states that the same result has been observed by M. Fahlberg. He 

 refers to Vetensk. Acad. ny. quart. 2. (1801.) 



