102 Mr. J. P. Joule on the Electric Origin 



menon as existing to a far greater extent when iron is used as 

 a positive element. 



16. A rod of iron and a small plate of platinized silver were 

 immersed in a dilute solution of sulphuric acid. On connect- 

 ing them with the galvanometer (5.)j the needle was perma- 

 nently deflected 292^°. After a few preliminary trials to ascer- 

 tain the proper point, I caused the needle to be maintained 

 by a glass weight at 55°, beyond which it was free to travel. 

 I then exposed the platinized silver to the air during one mi- 

 nute of time. On re-immersing it the needle sprang as far as 

 60°, and then immediately recoiled to its resting place at 55°, 

 thus indicating a transitory current of about 57|°. 



17. On exposing the platinized silver for 5" only, the trans- 

 itory current, ascertained in a manner similar to that just 

 mentioned, was 41°. 



18. Greater effects were obtained by washing and drying 

 the platinized silver before it was immersed. In this way the 

 needle, adjusted at 62°, would spring as far as QQ°, indicating 

 a transitory current of about 64°. Having now removed the 

 glass weight, the needle took up a permanent position at 29i°, 

 as at the beginning of the experiments. 



19. When, instead of the platinized silver, the positive ele- 

 ment (iron) was exposed to the air, whether simply or in con- 

 junction with washing and drying, no appreciable increase of 

 intensity was occasioned by its immersion. And although, on 

 the repetition of the experiment, I sometimes observed slight 

 effects, I conceive that they were owing to the power which 

 the negative element seems to possess of collecting upon its 

 surface the air held in solution by the circumambient liquid. 



20. With an arrangement of platinized silver and amalga- 

 mated zinc, I obtained results of a similar though less striking 

 character. The galvanometer indicated a permanent deflec- 

 tion of 62°, and after washing and drying the platinized silver, I 

 had a transitory deviation of 72°. The immersion of the amal- 

 gamated zinc, after washing and drying, produced no effect. 



21. The maximum effects of immersion were produced in 

 the following manner. A plate of silver was rubbed with a 

 little nitric acid, and then exj^osed to a red heat, by means of 

 which the film of nitrate of oxide of silver was decomposed 

 and metallic silver reduced*. When the plate prepared in 



* By tliis process all the oxygen is not driven off", but a considerable quan- 

 tity remains adhering to the silver so tenaciously that it is not entirely re- 

 moved by making the plate quite bright with glass paper. The oxj'geii 

 thus deposited (it can hardly, I think, be considered as chemically com- 

 bined with the silver) is the cause of the great intensity of the current im- 

 mediately after immersion. By simply heating the silver to redness the same 

 general effects can be produced, though not to the same extent 



