Sir Humphry Davy’s additional experiments , &c. 243 
the results, many of which have even surpassed my expec- 
tations. 
Sheets of copper, defended by from -i. to T ^ r o part of 
their surface of zinc, malleable and cast iron, have been 
exposed, for many weeks, in the flow of the tide in Ports- 
mouth Harbour, and their weights ascertained before and 
after the experiment. When the metallic protector was from 
.JL. to there was no corrosion nor decay of the copper ; 
with smaller quantities, such as from to ~ 5 , the copper 
underwent a loss of weight, which was greater in proportion 
as the protector was smaller ; and as a proof of the univer- 
sality of the principle, it was found that even T ioo part of cast 
iron saved a certain proportion of the copper. 
The sheeting of boats and ships, protected by the contact 
of zinc, cast and malleable iron in different proportions, 
compared with those of similar boats and sides of ships un- 
protected, exhibited bright surfaces, whilst the unprotected 
copper underwent rapid corrosion, becoming first red, then 
green, and losing a part of its substance in scales. 
Fortunately, in the course of these experiments, it has 
been proved that cast iron, the substance which is cheapest 
and most easily procured, is likewise most fitted for the pro- 
tection of the copper. It lasts longer than malleable iron, or 
zinc ; and the plumbaginous substance, which is left by the 
action of sea water upon it, retains the original form of the 
iron, and does not impede the electrical action of the remain- 
ing metal. 
I had anticipated the deposition of alkaline substances in 
certain cases upon the negatively electrical copper. This 
has actually happened. Some sheets of copper, that have 
