Two Fluids, and of Two Metals not in Contact. 545 



I have communicated these details in order to show that the 

 phenomena in circuits of the kind described are by no means 

 ahvays so simple as it seems they should be according to the 

 affinity-theory. That, moreover, they do not speak for it is 

 undoubtedly evident enough. Only at first and transitorily has 

 the sulphuric acid the ascendency over the iodide of potassium ; 

 subsequently this acid, although it incontrovertibly attacks the 

 zinc more violently than the iodide of potassium (even with 

 energy increasing during the course of the action), is always 

 overpowered by it ; nay, what is remarkable, the less dilute acid 

 (with 4 times its vol. of water) succumbs to the iodide of potas- 

 sium to a greater extent than the acid diluted with 9 times its 

 vol. of water. How is all this to be explained from the relative 

 affinity of the oxygen and iodine to the zinc ? 



The above facts appear to stand in contradiction to the ex- 

 periment of Faraday, mentioned at p. 487^ in which a current 

 was obtained with sulphuric acid, strongly overpowering the 

 iodide of potassium. However, this contradiction is merely ap- 

 parent ; for, what was not there observed, the sulphuric acid was 

 not pure, but purposely mixed with some nitric acid. A sulphuric 

 acid, containing nitric acid, has in fact, (of which I have con- 

 vinced myself,) in a high degree the superiority over the iodide 

 of potassium ; a powerful and permanent deflexion in favour of 

 5 :> i is immediately obtained, and at the same time there may 

 be distinctly observed, from the yellow coloration of the solution 

 of the iodide of potassium around the platina plate, the separa- 

 tion of the iodine*. 



If, nevertheless, this fact is still employed, after so many 

 proofs against the affinity-theory, as an argument in favour of 

 it, it may then with justice be asked, Why then does the sul- 



* A dilute sulphuric acid, consisting of 1 vol. acid of 1"827 spec, gr., and 

 4 vol. water, to which was added an eighth of its volume of nitric acid of 

 1'321 spec, gr., was used for this experiment. A moi*e dilute mixture, con- 

 sisting of 12 parts in weight of dilute sulphuric acid (1 vol. concent, acid, and 

 9 vol. water), and one part in weight of the above nitric acid, gave by far 

 Aveaker results. Platina, combined with zinc, produced, it is true, a current 

 in the direction s > i ; but even after it had been heated, it did not equal the 

 current from silver-zinc or copper-zinc. With the combination tin-zinc the 

 current had the direction s < i, as with pure sulphuric acid. 



Dilute nitric acid alone (p. 489) likewise gives rise to similar effects. Copper, 

 silver, heated platina, combined with zinc, immediately produced a powerful 

 current in the direction s:::^ i. With non-heated platina the direction of the 

 deflexion was the same, but the intensity only slight, merely 5° ; it increased, 

 however, perceptibly, and without any oscillation the needle slowly rose to 

 45', where it remained. With tin the direction of the current was the reverse, 

 i. e. the iodide of potassium had the superiority, and indeed strongly. This is 

 the more remarkable, as I convinced myself that the tin in the same acid 

 is highly negative to the zinc. 



