on the Preservation of Metals by Electro-chemical Means. 95 
metal. I could not produce any protecting action of zinc or 
iron upon copper through the thinnest stratum of air, or the 
finest leaf of mica, or of dry paper; but the action of the me- 
tals did not seem to be much impaired by the ordinary ccating 
of oxide or rust; nor was it destroyed when the finest bibulous 
or silver paper, as it is commonly called, was between them, 
being moistened with sea water. I made an experiment with 
different folds of this paper. Pieces of copper were covered 
with one, two, three, four, five and six folds; and over them 
were placed pieces of zinc, which were fastened closely to them 
by thread: each piece of copper so protected was exposed in a 
vessel of sea water, so that the folds of paper were all moist. 
It was found, in the case in which a single leaf of paper was 
between the zinc and the copper, there was no corrosion of 
the copper; in the case in which there were two leaves, there 
was a very slight effect; with three, the corrosion was distinct ; 
and it increased, till with the six folds the protecting power. 
appeared to be lost: and in the case of the single leaf there 
was this difference from the result of immediate contact, that 
there was no deposition of earthy matter :—showing that there 
was no absolute minute contact of the metals through the 
moist paper ; which was likewise proved by other experiments: 
for a thin plate of mica, as I haye just mentioned, entirely de- 
stroyed the protecting effect of zinc; and yet when a hole was 
made in it, so as to admit a very thin layer of moisture be- 
tween the zinc and copper, the corrosion of the copper, though 
not destroyed, was considerably diminished. 
The rapid corrosion of iron and zinc, particularly when 
used to protect metals, only in very small quantities, induced 
me to try some experiments as to their electro-chemical powers 
in menstrua out of the contact, or to a certain extent removed 
from the contact of air, such as might be used for moistening 
paper under the copper sheathing of ships. The results of 
these experiments I shall now detail. A small piece of iron 
was placed in one glass filled with a saturated solution of 
brine, which contains little or no air; copper, attached by a 
wire to the iron, was placed in a vessel containing sea water, 
which was connected with the brine by moistened tow. The 
copper did not corrode, and yet the iron was scarcely sensibly 
acted upon, and that only at the surface of the brine; and a 
much less effect was produced upon it in many weeks than 
would have been occasioned by sea water in as many days. 
With zinc and brine in the same kind of connexion there 
was a similar result; but the solution of the zinc was com- 
paratively more rapid than that of the iron, and the copper 
was 
