96 Sir H. Davy on the Corrosion of [Aug. 



upon it would be null ; and whatever might be the differences of 

 the kinds of copper sheeting and their electrical 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 applicable in practice. I next thought of 

 the contact of zinc, tin, or iron : but I was for some time pre- 

 vented 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, however, 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 chemical action would be destroyed by a very feeble 

 electrical force, I resolved to try some experiments on the sub- 

 ject. I began with an extreme case. 1 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 l-20th of the surface of the copper. Examined after three 

 days, the copper remained perfectly clean, while 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 considerable corrosion of 

 the copper, and a distinct blue tint in the liquid. 



If l-20th part of the surface of tin prevented the action of sea 

 water rendered slightly acidulous by sulphuric acid, I had no 

 doubt that a much smaller quantity would render the action of 

 sea water, which depended only upon the loosely attached oxy- 

 gene of common air, perfectly null ; and on trying 1 -200th 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 instituted 

 a number of experiments, in most of which I was assisted by 

 Mr. Faraday, to ascertain all the circumstances connected with 

 the preservation of copper by a more oxidable metal. I found, 

 that whether the tin was placed either in the 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 zinc or iron, whether malleable or cast, no such diminu- 

 tion of effect was produced. The zinc occasioned only a white 

 cloud in the sea water, which speedily sunk to the bottom of 

 the vessel in which the experiment was made. The iron occa- 

 sioned a deep orange precipitate ; but after many weeks, not 

 the smallest portion of copper was found in the water ; and so 



