244 Sir Humphry Davy's additional experiments 
been exposed nearly four months to the action of sea water, 
defended by from ~ to ~ of their surface of zinc and iron, 
have become coated with a white matter, which, on analysis, 
has proved to be principally carbonated lime, and carbonate 
and hydrate of magnesia. The same thing has occurred with 
two harbour boats, one of which was defended by a band of 
zinc, the other by a band of iron, equal to about ~ of the 
surface of the copper. 
These sheets and boats remained perfectly clean for many 
weeks, as long as the metallic surface of the copper was ex- 
posed ; but lately, since it has become coated with carbonate 
of lime and magnesia, weeds have adhered to these coatings, 
and insects collected on them ; but on the sheets of copper, de- 
fended by quantities of cast iron and zinc, bearing a proportion 
below the electrical power of the copper being less nega- 
tive, more neutralised, and nearly in equilibrio with that of 
the menstruum, no such effect of deposition of alkaline matter 
or adherence of weeds has taken place, and the surface, 
though it has undergone a slight degree of solution, has re- 
mained perfectly clean : a circumstance of great importance, 
as it points out the limits of protection ; and makes the appli- 
cation of a very small quantity of the oxidable metal, more 
advantageous in fact than that of a larger one. 
The wear of cast iron is not so rapid ; but that a mass of 
two or three inches in thickness will last for some years. At 
least the consumption in experiments which have been going 
on for nearly four months, does not indicate a higher ratio. 
This must however depend on the relation of its mass to that 
of the copper, and upon other circumstances not yet ascer- 
tained ( such as temperature, the relative saltness of the sea, 
