280 Proceedings of the Royal Society. 
Boats, and the sides of ships, protected in this way, were also 
similarly preserved. 
Of the different protecting metals, cast iron is most convenient, 
and the plumbaginous substance formed upon it does not impede 
its electrical action. The President formerly anticipated the de- 
position of earthy substances upon the negative copper, and this he 
now found to take place upon sheets of copper exposed about four 
months to sea water, and defended by from ;/, to J their surface 
of zinc and iron; they became coated with carbonate of lime and 
magnesia; but this effectis easily prevented, by duly diminishing 
the proportion of the protecting metal, so as to prevent the excess 
of negative power in the copper which then remains bright and clean. 
The author observed that many singular facts had occurred in 
the course of his researches, some of which bore upon general 
science. Weak solutions of salt act strongly upon copper, but 
strong ones do not affect it, apparently because they contain little air, 
the oxygen of which seems necessary to give the electro-positive 
power to these menstrua. Upon the same principle, alkaline solu- 
tions and lime-water prevent the action of sea-water on copper, 
having in themselves the positive electrical energy which renders 
the copper negative. 
The President concluded this paper with some further applica- 
tions of electro-chemical theory to the subject of it, and referred to 
the principles developed, as suggesting means of preserving instru- 
ments of brass and of steel, by iron and by zinc, a circumstance 
already taken advantage of by Mr. Pepys, in enclosing delicate 
cutting instruments in handles or cases lined with zinc. 
The Society then adjourned for the long vacation, 
