338 Sir Humphry D avy*s further researches on 
tinct : 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 have just 
mentioned, entirely destroyed the protecting effect of zinc ; 
and yet when a hole was made in it, so as to admit a very 
thin layer of moisture between the zinc and copper, the cor- 
rosion 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, in- 
duced me to try some experiments as to their electro-che- 
mical 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 connection there 
was a similar result : but the solution of the zinc was com- 
