544 DR. DAVY ON THE TORPEDO. 



decompositions ; and several other trials which I have made are amply confirmatory 

 of this. In these latter experiments I have not, as in the former, coated the wires 

 introduced into the fluids with sealing-wax, leaving the points only exposed. Though 

 the wires were naked, and in every instance introduced more than a quarter of an 

 inch into the fluid, and the distance between them was at least a tenth of an inch, 

 yet satisfactory results were obtained. Using either a saturated solution of common 

 salt, or a mixture of equal parts of sulphuric acid of commerce and water, and 

 platina or gold wires, gas was given off" round each wire under the influence of the 

 discharge of the electricity of an active fish, one contact wire being applied to the 

 under surface, and the other to the upper surface of the Torpedo. When steel needles 

 were used with the salt water, then gas was disengaged only from the one in con- 

 nexion with the under surface of the fish, the other needle becoming oxidated. Using 

 a strong solution of nitrate of silver and gold wires, silver was precipitated only on 

 that in connexion with the under surface ; employing strong nitric acid and platina 

 wires, gas was given off from one only, that in connexion with the upper surface ; 

 and using a solution of iodide of potassium and starch*, the iodine in combination with 

 the starch, as indicated by the discolouration, was precipitated round the same wire. 



Even the decomposition of water has been effected when the circle has been in- 

 terrupted by four portions of the solution of common salt, contained in small tubes 

 with two needles in each, the needles in one connected with those in the other, and 

 at the same time with the galvanometer, a spiral holding an unmagnetized needle 

 and an air thermometer. And simultaneous with the chemical decomposition, the 

 needle in the galvanometer has been moved, and the spirit in the air thermometer 

 has been raised, and the needle in the spiral has been magnetized. 



The tests or indications of the electricity of the Torpedo at present known are six 

 in number, namely, the physiological effect, as the sensation it imparts is sometimes 

 called ; the chemical effects, as the precipitation of iodine, the decomposition of 

 water, &c. ; its effect on the thermometer, on the galvanometer, and on steel in the 

 spiral. These different tests, in point of delicacy, I am inclined to believe are in the 

 order in which they are enumerated. That the two first should be placed highest, 

 and that sensation should have the precedence, the experiments which I have made 

 appear to prove, independently of all analogy. 



When the human body has formed part of a circle of communication between the 

 two opposite surfaces of a Torpedo, and also a chemical apparatus with platina wires 

 and the solution of iodide of potassium and starch, the shock experienced by the 

 hands has been strong, and the chemical effect either null or slight, no gas appearing 

 when a strong solution of salt has been used, and no precipitation of iodine occurring 

 unless the platina points were very nearly in contact, and the fish energetic. 



* When starch in powder is added to a saturated or nearly saturated solution of the iodide of potassium, a 

 transparent gelatinous mass is formed. This I have used in my experiments ; a single combination of copper 

 and zinc wire, acted on by a very dilute acid, occasions in this compound a precipitation of iodine. 



