32 Sir H. Davy on the Corrosion 



upon sea-water when in a positive state ; and, consequently, 

 if it could be rendered slightly negative, the corroding action 

 of sea-water upon it would be null ; and whatever might be 

 the differences of the kinds of copper sheeting and their elec- 

 trical action upon each other, still every effect of chemical 

 action must be prevented, if the whole surface were rendered 

 negative. But how was this to be effected ? I at first thought 

 of using a Voltaic battery ; but this could be hardly applica- 

 ble in practice. I next thought of the contact of zinc, tin, or 

 iron : but I was for some time prevented from trying this, by 

 the recollection that the copper in the Voltaic battery, as well 

 as the zinc, is dissolved by the action of diluted nitric acid; 

 and by the fear that too large a mass of oxidable metal would 

 be required to produce decisive results. After reflecting, how- 

 ever, for some time on the slow and weak action of sea-water 

 on copper, and the small difference which must exist between 

 their electrical powers ; and knowing that a very feeble che- 

 mical action would be destroyed by a very feeble electrical 

 force, I resolved to try some experiments on the subject. I 

 began with an extreme case. I rendered sea-water slightly 

 acidulous by sulphuric acid, and plunged into it a polished 

 piece of copper, to which a piece of tin was soldered equal to 

 about one-twentieth of the surface of the copper. Examined 

 after three days the copper remained perfectly clean, whilst 

 the tin was rapidly corroded : no blueness appeared in this 

 liquor; though, in a comparative experiment, when copper 

 alone and the same fluid mixture was used, there was a consi- 

 derable corrosion of the copper, and a distinct blue tint in the 

 liquid. 



If one-twentieth part of the surface of tin prevented the 

 action of sea-water rendered slightly acidulous by sidphuric 

 acid, I had no doubt that a much smaller quantity would ren- 

 der the action of sea-water, which depended only upon the 

 loosely attached oxygen of common air, perfectly null ; and 

 on trying jfo part of tin, I found the effect of its preventing 

 the corrosion of the copper perfectly decisive. 



5. This general result being obtained, I immediately insti- 

 tuted a number of experiments, in most of which I was as- 

 sisted by Mr. Faraday, to ascertain all the circumstances con- 

 nected with the preservation of copper by a more oxidable 

 metal. I found, that whether the tin was placed either in tne 

 middle, or at the top, or at the bottom of the sheet of copper, 

 its effects were the same ; but, after a week or ten days, it 

 was found that the defensive action of the tin was injured, a 

 coating of sub-muriate having formed, which preserved the 

 tin from the action of the liquid. 



With 



