192 THE PRESERVATION OF 
Davy admitted the theory of contact, that is, the production of electricity 
by the contact of two metals, resulting from mutual action, Chemical action, 
according to him, only served for the transmission of electricity from one body 
to another. This view prevented him from deducing from his discovery con- 
sequences which naturally flow from it. His first statement was, that a piece 
of zinc of the size of a pea, or of the point of an iron nail, was quite sufficient 
to preserve from 40 to 50 square inches of copper wherever placed, and that a 
little piece of zine having been fixed on top of a piece of copper, and a much 
larger bit of iron below it, and the whole immersed in salt water, the cop- 
per not only was preserved on both sides, but the iron also, which, after a 
fortnight, had kept its brightness equally as well as the other metal. He con- 
cluded from this, at once, that small quantities of zinc, of iron, or of cast-iron, 
when placed in contact with the copper-sheathing of vessels, prevented its cor- 
rosion. He added, besides, that, since negative electricity could not be regarded 
as favorable to animal or vegetable life, because it caused the precipitation of 
magnesia upon copper, a substance very prejudicial to land plants, this elee- 
tricity ought to help to keep the bottoms of vessels clean. 
The lords of the admiralty having furnished him with the means of experi- 
menting on a large scale with his mode of preserving the copper-sheathing of 
-vessels at Chatham and Portsmouth, he established the following facts :* Sheets 
of copper in contact with zine, iron or cast-iron, over 25 or gq/oq part of their 
surface, having been exposed for several weeks in the harbor of Portsmouth to 
the action of the tide, and their weights having been determined before and 
after the experiment, Davy found that when the metallic protector covered a 
surface of from =}, to ;4,5 of the copper, neither corrosion nor diminution of the 
latter metal took place, but when the ratio was from 5345 to zi the copper under- 
went a loss of weight greater in proportion as the protection was smaller. He 
considered cast-iron, a substance so readily and cheaply found everywhere, as 
the best and the most appropriate for the protection of copper, and as lasting 
as long as iron and zine. 
The sheets of copper of two small vessels, thus protected, were kept per- 
fectly clean for several weeks, as long as the metallic surface of the copper 
had remained uncovered; but as soon as the metal was covered with carbonate 
of lime and magnesia, plants and insects collected there. 
Again, we find the following facts in the Philosophical Transactions of Lon- 
don tor 1825, pp. 340 et seq.: 
‘*The first experiment of this kind was tried on the Sammarang, of 238 guns, in March, 
1824, and which had been coppered three years before in India. When she came into dock 
at that time, before she was protected, she was covered with thick green carbonate and sub- 
muriate of copper, and with a number of long weeds, principally fuci, and a quantity of 
zodphites, adhering to different’ parts of the bottom. For the purpose of protection, Davy 
employed cast-iron, equal in surface to about 75 of that of the copper, which was applied in 
four masses, two near the stern, two on the bows. She made a voyage to Nova Scotia, and 
returned in January, 1825. When she was again brought into dock, there was not the 
smallest weed or shell-fish upon the whole of the bottom from a few feet round the stern-pro- 
tectors to the lead on her bow. Round the stern-protectors there was a slight adhesion of 
rust of iron, and upen this there were some zoéphites of the capillary kind, of an inch and 
a half or two inches in length, and a number of minute barnacles, both Lepas unatifera and 
Balanus tintinnabulum. Jor a considerable space round the protectors, both on the stern 
and bow, the eopper was bight; but the color became green towards the central parts of 
the ship; yet even here the rust or verdigris was a light powder, and only small in quan- 
tity, and did not adhere, or come off, in scales, and there had been evidently little copper 
lost in the voyage. 
“The yacht Elizabeth was protected by about +45 part of malleable iron placed in two 
masses in the stern. She had been occasionally employed in sailing, and had been some- 
times in harbor, during six months. When Davy saw her, at the end of this time, she was 
periectly clean, and the copper apparently untouched. Her owner informed him that there 
* Philosophical Transactions, 1824; Annales de Chimie et de Physique, t. xxix, p. 187. 
