338 



Dr. G. Gore. On some Relations of [Feb. 21, 



solid deposit containing black suboxide of magnesium formed upon 

 the positive plate only.* And with aluminium in solution of 

 iodic acid a compound of iodine was absorbed, and both the plates 

 increased about 16 per cent, in weight.f In seven cases (Nos. 

 6, 7, 14, 28, 65, 67, 81), from causes which were not very apparent, 

 the comparison sheets lost weight more than the positive ones. The 

 considerable degree of variability of rate of ordinary corrosion 

 detracted greatly from the usefulness of the comparison sheet. 



The results show that the proportion of loss of the positive plate by 

 " local action " to that by corrosion attending external current varied 

 greatly in different cases ; with silver in a dilute solution of ferric 

 sulphate, the latter amounted to only about 1*3 per cent, of the total 

 corrosion, whilst with tin in weak hydrochloric acid it was as much as 

 95*25 per cent. In no instance was the whole of the metal dissolved 

 by "local action," and in no case was the whole of the corrosion 

 attended by external current. 



The amount of corrosion of the positive plate was in nearly all 

 cases greater than that of the comparison one. Six cases were 

 selected from the table, in which the apparent corrosion of the latter 

 sheet was the greatest, and some trouble was taken to ascertain by 

 experiment whether either of these was a real one. After varying the 

 conditions of the trial in several ways, with pure copper in a mixture 

 of 50 minims of pure nitric acid per ounce of distilled water, it was 

 found that in this instance the comparison plate was always corroded 

 the fastest. 



As the amount of corrosion of the positive plate was in nearly all 

 cases greater than that of the comparison one, and the proportion of 

 gas to corrosion was also frequently less with the former than with 

 the latter, a solid deposit was usually more readily formed on the 

 positive plate than upon the comparison one, and the chemical 

 products were not always exactly the same upon the two plates. An 

 instance of this kind occurred with amalgamated silver in a solution 

 of potassic cyanide ; the positive plate alone became brown. As the 

 chemical products of " local action," and of corrosion attended by 

 external current, are usually the same, in cases where they are 

 different, though resulting from the same materials, the total expendi- 

 ture of potential molecular motion would probably be different, 

 because the quantities of residual molecular energy of the ingredients 

 would not be the same. 



The proportion of corrosion of the positive plate accompanying 

 external current, to that produced by "local action," may be approxi- 

 mately arrived at, either by deducting from the total amount of 

 corrosion of that plate the loss by weight of the comparison one, or by 



* See "Proc. Birm. Phil. Soe.," vol. iv. 

 f Ibid., vol. iv, Part I, p. 65. 



