1825.] Sir H. Davy on Copper Sheathing. 299 



rapidity of the motion of tlie ship ;) circumstances in relation to 

 which I am about to make decisive experiments. 



Many singular facts have occurred in the course of these 

 researches. I shall mention some of them, that I have confirmed 

 by repeated experiments, and which have connexions with 

 general science. 



Weak solutions of salt act strongly upon copper; strong ones, 

 as brine, do not affect it ; and the reason seems to be, that they 

 contain little or no atmospheric air, the oxygen of which seems 

 necessary to give the electro-positive principle of change to 

 menstrua of this class. 



I had anticipated the result of this experiment, and upon the 

 same principle of some others. 



Alkaline solutions, for instance, impede or prevent the action 

 of sea water on copper; having in themselves the positive elec- 

 trical 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 produce an equilibrium in the electrical powers, the agency 

 of all combinations formed of metals and fluids is to occasion 

 decompositions, in such an order that alkaline, metalhc, and 

 inflammable matters are determined to the negative part of the 

 combination, and chlorine, iodine, oxygene and acid matters to 

 the positive part. I have shown in the Bakerian Lecture for 

 1806, that this holds good in the Voltaic battery. 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 carbonate of soda will gradually 

 accumulate till the whole surface in the air is covered with its 

 crystals : — and if the iron is in one vessel, and the copper form- 

 ing an arc with it in another ; and a third vessel of sea water in 

 electrical connexion by asbestos 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. 



1 shall not take up the time of the Society by referring to 

 some obvious practical applications of these researches, to the 



f)reservation of finely divided astronomical instruments of brass 

 )y iron, of instruments of steel by iron, or zinc : my friend Mr. 

 J^cpys has already ingeniously taken advantage of this last cir- 

 cumstance, in inclosing finely cutting instruments in handles or 

 cases lined with zinc, and many other such applications will 

 occur. I cannot C(3nclude, without nienlioniug particularly my 



• I am at present engaged in applying this principle to cxperiiaentti on the preserva. 

 tion of animal and vegetable substances. 



