420 Sir Humphry Davy on the relations of 
the world has proved the full efficacy of the electro-chemical 
means of preserving metals, particularly the copper sheathing 
of ships ; but a hope I had once indulged, that the pecu- 
liar electrical state would prevent the adhesion of weeds or 
insects has not been realized ; protected ships have often 
indeed returned after long voyages perfectly bright,* and 
cleaner than unprotected ships, yet this is not always the 
case ; and though the whole of the copper may be preserved 
from chemical solution in steam vessels by these means, yet 
they must be adopted in common ships only, so as to pre- 
serve a portion,— so applied as to suffer a certain solution of 
the copper ; *f* and an absolute remedy for adhesions, is to 
be sought for by other more refined means of protection, 
and which appear to be indicated by these researches. 
The nails used in ships are an alloy of copper and tin, which 
I find is slightly negative with respect to copper, and it is on 
these nails that the first adhesions uniformly take place : a 
slightly positive and slightly decomposable alloy would pro- 
bably prevent this effect, and I have made some experiments 
favourable to the idea. 
In general, all changes in metals which would indicate the 
power of chemical attraction, are easily determined by elec- 
trical means. Thus I found copper hardened by hammering 
* The Carnebrea Castle. 
f A common cause of adhesions of weeds or shell fish, is the oxide of iron 
formed and deposited round the protectors. In the only experiment in which zinc 
has been employed for this purpose in actual service, the ship returned after two 
voyages to the West Indies, and one to Quebec, perfectly clean. 
The experiment was made by Mr. Lawrence, of Lombard-street, who in his 
letter to me states that the rudder, which was not protected, had corroded in the 
usual manner. 
