corrosion of copper sheeting by sea water , &c. 155 
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 considerable 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 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 oxygene of common air, perfectly null ; 
and on trying ~ ^ part of tin, I found the effect of its pre- 
venting 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 
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 l'quid. 
With zinc or iron, whether malleable or cast, no such di- 
minution 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 occasioned a deep orange precipitate : but after many 
weeks, not the smallest portion of copper was found in the 
