94 Sir H. Davy’s further Researches 
hasty deposition of earthy and alkaline matter, and crystals of 
carbonate of soda adhered to the copper at the surface where 
it was clean and bright; but in the lower part it was coated 
with revived metallic zinc. In the second glass the wire was 
covered over with fine crystals of carbonate of lime; and the 
same phenomenon of the separation of carbonate of soda oc- 
curred, but in a less degree. In the third glass the wire was 
clean, but without depositions; and the presence of alkaline 
matter could only be distinguished by chemical tests. In the 
fourth glass the copper was bright, evidently in consequence 
of a slight but general corrosion, but with a scarcely sensible 
deposit; in the fifth, the deposit was very visible; and in the 
seventh the wire was covered with green rust. 
These results, which showed that a very small quantity only 
of the imperfect or fluid conductor was sufficient to transmit 
the electrical power, or to complete the chain, induced me to 
try if copper nailed upon wood, and protected merely by zinc 
or iron on the under surface, or that next the wood, would 
not be defended from corrosion. For this purpose I covered 
a piece of wood with small sheets of copper, a nail of zinc of 
about the 1-200th part.of the surface of the copper being pre- 
viously driven into the wood: the apparatus was plunged in 
a large jar of sea water: it remained perfectly bright for many 
-weeks; and when examined, it was found that the zinc had 
only suffered partial corrosion, that the wood was moist, and 
that on the interior of the copper there was a considerable 
portion of revived zinc, so that the negative electricity, by its 
operation, provided materials for its future and constant ex- 
citement. In several trials of the same kind, iron was used 
with the same results; and in all these experiments there ap- 
peared to be this peculiarity in the appearance of the copper, 
that unless the protecting metal below was in very large mass, 
there were no depositions of calcareous or magnesian earths 
upon the metal; it was clean and bright, but never coated. 
The copper in these experiments was nailed sometimes upon 
paper, sometimes upon the mere wood, and sometimes upon 
- linen; and the communication was partially interrupted be- 
tween the external surface and the internal surface by cement ; 
but even one side or junction of a sheet seemed to allow suf- 
ficient communication between the moisture on the under sur- 
face and the sea water without, to produce the electrical effect 
of preservation. 
These results upon perfect and imperfect conductors led to 
another inquiry, 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 preserving 
metal. 
