the preservation of metals by electro-chemical means. 331 
over with carbonate of lime and magnesia : a sheet of copper 
of the same size protected by gained only 1 drachm in 
the same time, and a part of it was green from the adher- 
ing salts of copper ; whilst an unprotected sheet of the same 
class, both as to size and weight, and exposed for the same 
time, and as nearly as possible under the same circumstances, 
had lost 14 drachms ; but experiments of this kind, though 
they agree when carried on under precisely similar circum- 
stances, must of necessity be very irregular in their results, 
when made in different seas and situations, being influenced 
by the degree of saltness, and the nature of the impregna- 
tions of the water, the strength of tide and of the waves, 
the temperature, &c. 
In examining sheets which had been defended by small 
quantities of iron in proportions under ~~ and above rood t 
whether they were exposed alone, or on the sides of boats, 
there seemed to me no adhesions of conferva, except in cases 
where the oxide of iron covered the copper immediately 
round the protectors ; and even in these instances such adhe- 
sions were extremely trifling, and might be considered rather 
as the vegetations caught by the rough surface of the oxide 
of iron, than as actually growing upon it. 
Till the month of July 1824 all the experiments had been 
tried in harbour, and in comparatively still water; and though 
it could hardly be doubted, that the same principles would 
prevail in cases where ships were in motion, and on the ocean ; 
yet still it was desirable to determine this by direct experi- 
ment ; and I took the opportunity of an expedition intended 
to ascertain some points of longitude in the north seas, and 
which afforded me the use of a steam boat, to make these 
MDCCCXXV. X x 
