the preservation of metals by electro-chemical means. 337 
calcareous or magnesian earths upon the metal ; it was clean 
and bright, but never coated. The copper in these experi- 
ments was nailed sometimes upon paper, sometimes upon the 
mere wood, and sometimes upon linen ; and the communica- 
tion was partially interrupted between the external surface 
and the internal surface by cement ; but even one side or 
junction of a sheet seemed to allow sufficient communication 
between the moisture on the under surface and the sea water 
without, to produce the electrical effect of preservation. 
These results upon perfect and imperfect conductors led to 
another enquiry, important as it relates to the practical appli- 
cation of the principle ; namely, as to the extent and nature 
of the contact or relation between the copper and the pre- 
serving 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 metals did not seem to be much impaired by the ordi- 
nary coating 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 cop- 
per 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 
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 dis- 
