Sir Humphry. Davy's further researches, &c. 
and carbonate of copper and carbonate of magnesia forms 
upon it, to which weeds and shell fish adhere. 
As long as the whole surface of the copper changes or 
* 
corrodes, no such adhesions can occur ; but when this green 
rust has partially formed, the copper below is protected by i , 
and there is an unequal action produced, the electrical effect of 
the oxide, submuriate, and carbonate of copper formed, be- 
ing to produce a more rapid corrosion of the parts still exposed 
to sea water ; so that the sheets are often found perforated 
with holes in one part, after being used five or six years, and 
comparatively sound in other parts. 
There is nothing in the poisonous nature of the metal 
which prevents these adhesions. It is the solution by which 
they 2iYe prevented — the wear of surface. Weeds and shell fish 
readily adhere to the poisonous salts of lead which form 
upon the lead protecting the fore part of the keel ; and 
to the copper, in any chemical combination in which it is 
insoluble. 
In general in ships in the navy the first effect of the adhe- 
sion of weeds is perceived upon the heads of the mixed metal 
nails, which consist of copper alloyed by a small quantity of 
tin. The oxides of tin and copper which form upon the 
* 
head of the nail and in the space round it, defend the metal 
from the action of sea water ; and being negative with respect 
to it, a stronger corroding effect is produced in its immediate 
vicinity, so that the copper is often worn into deep and irre- 
gular cavities in these parts. 
When copper is unequally worn, likewise in harbours or 
• seas where the water is loaded with mud or mechanical de- 
posits, this mud or these deposits rest in the rough parts or 
