Producible by Attrition and Contact of Metals. 29 



was inserted a platinum wire, one sealed in the glass, the other confined by 

 means of a cork. The platinum wires being connected with the binding screws, 

 the wheel was made to revolve, witli great rapidity, during an hour. At the 

 end of this time there was not the slightest symptom of decomposition of the 

 iodide. I also tried the experiment with turmeric paper moistened with solu- 

 tion of iodide of potassium, with no greater effect ; although a current of com- 

 mon electricity, which would not have affected the galvanometer, produced a 

 brown spot when passed through the wires for a few moments. Other asso- 

 ciations of metals had no better success. 



I then made an experiment which, if previously tried, would have shown 

 me the impracticability of producing any decomposition by these means; it was 

 as follows: — Tlie tube with platinum wires, being cleaned out, was filled with 

 salt water, and made a part of the circuit between the bismuth ring and anti- 

 mony rubber. The galvanometer was also introduced into the circuit. The 

 bismuth ring was now thrown into rapid revolution ; but there was not the 

 slightest deflection of the needle, although the platinum wires were separated 

 from each other only one-fortieth of an inch: even this small extent of salt 

 water could not be traversed by the deflecting agent. 



To contrast the easy passage of the ordinary voltaic agent through salt water 

 with the impenetrability of this liquid to the deflecting influence generated by 

 attrition of metals, I removed the tube containing platinum wires, and substi- 

 tuted for it a glass tube forty inches long, filled with salt water, and stopped 

 at both ends with corks, through which passed a platinum wire long enough 

 to touch the water within. A zinc and a copper plate, each half an inch square, 

 acted on by water containing as much sulphuric acid as imparted to it a scarcely 

 discoverable taste, were brought into connexion with the copper wires of the 

 long glass tube, in the usual manner ; the galvanometer being included in the 

 circuit. The moment the connexions were established, the needle moved 

 slowly, and settled at 60', proving that this deflecting agent traversed IHOO 

 times the length of liquid, which the other deflecting agent refused to enter at 

 all, and perhaps it would have passed through a hundred times even that 

 extent. 



To render this result more striking, the last experiment was repeated, 

 with the single difference of substituting spring water for water acidulated 

 with sulphuric acid. On establishing the connexions, the needle moved 30°: 



