206 Sir H. Davy on Preserving the Copper Sheathing of Ships. 



Alkaline solutions, for instance, impede or prevent the ac- 

 tion of sea-water on copper, having in themselves the positive 

 electrical energy which renders the copper negative. Lime- 

 water even, in this way renders null the power of action of 

 copper on sea-water*. 



The tendency of electrical and chemical action being always 

 to jM-oduce an equilibrium in the electrical powers, the agency 

 of all combinations formed of metals and fluids is to occasion 

 decomposition, in such an order that alkaline, metallic, and 

 inflammable matters are determined to the negative part of 

 the combination ; and chlorine, iodine, oxygen, and acid 

 matters to the positive part. I have shown in the Bakerian 

 lecture for 1806, that this law holds good in the Voltaic bat- 

 tery. The same- law applies to these feebler combinations. 

 If copper in contact with cast iron be placed in a vessel half 

 full of sea-water, and having its surface partially above that 

 of the water, it will become coated with carbonate of lime, 

 carbonate of magnesia, and carbonate of soda ; and the car- 

 bonate of soda will gradually accumulate till the whole sur- 

 face in the air is covered with its crystals : — and if the iron is 

 in one vessel, and the copper forming an arc with it in another, 

 and a third vessel of sea-water in electrical connexion by as- 

 bestos or cotton is intermediate, the water in this intermediate 

 vessel continually becomes less saline ; and undoubtedly, by a 

 continuance of the process, might be rendered fresh. 



I shall not take up the time of the society by referring to 

 some obvious practical applications of these researches to the 

 preservation of finely divided astronomical instruments of brass 

 by iron, of instruments of steel by iron or zinc : my friend 

 Mr. Pepys has already ingeniously taken advantage of this 

 last circumstance, in inclosing finely cutting instruments in 

 handles or cases lined with zinc, and many other such appli- 

 cations will occur. I cannot conclude without mentioning 

 particularly my obligations to Sir Byam Martin, the Comp- 

 troller, and Sir Robert Seppings, the Surveyor of the Navy, 

 for the interest they have taken and the zeal they have shown 

 in promoting these researches; and without stating how much 

 I owe to the care, attention, and accuracy of Mr, Nolloth, 

 master shipwright, and Mr. Goodrich, mechanist in the Dock- 

 yard at Portsmouth, in superintending the execution of many 

 of the experiments. 



• I am at present engaged in applying this principle to experiments on 

 the preservation of animal and vegetable substances. 



XXXV. 



