Sir H. Davy on Corrosion of Copper Sheeting by Sea-water. 31 



which constantly accumulates ; at the same time that the sur- 

 face of the copper corrodes, appearing red in the water, and 

 grass-green where it is in contact with air. Gradually car- 

 bonate of soda forms upon this grass-green matter ; and these 

 changes continue till the water becomes much less saline. 



The green precipitate, when examined by the action of so- 

 lution of ammonia and other tests, appears principally to con- 

 sist of an insoluble compound of copper (which may be con- 

 sidered as a hydrated sub-muriate) and hydrate of magnesia. 



According to the views which I developed fourteen years 

 ago, of the nature of the compounds of chlorine, and which 

 are now generally adopted, it is evident that soda and mag- 

 nesia cannot appear in sea-water by the action of a metal, un- 

 less in consequence of an absorption or transfer of oxygen. 

 It was therefore necessary for these changes, either that water 

 should be decomposed, or oxygen absorbed from the atmo- 

 sphere. I found that no hydrogen was disengaged, and con- 

 sequently no water decomposed : necessarily, the oxygen of 

 the air must have been the agent concerned, which was made 

 evident by many experiments. 



Copper in sea-water deprived of air by boiling or exhaus- 

 tion, and exposed in an exhausted receiver or an atmosphere 

 of hydrogen gas, underwent no change ; and an absorption in 

 atmospherical air was shown when copper and sea-water were 

 exposed to its agency in close vessels. 



4. In the Bakerian Lecture for 1806, I have advanced the 

 hypothesis, that chemical and electrical changes may be iden- 

 tical, or dependent upon the same property of matter; and I 

 have further explained and illustrated this hypothesis in an 

 elementary work on Chemistry published in 1812. Upon this 

 view, which has been adopted by M. Berzelius and some other 

 philosophers, I have shown that chemical attractions may be 

 exalted, modified, or destroyed, by changes in the electrical 

 states of bodies ; that substances will only combine when they 

 are in different electrical states ; and that, by bringing a body 

 naturally positive artificially into a negative state, its usual 

 powers of combination are altogether destroyed; and it was 

 by an application of this principle that in 1807 I separated 

 the bases of the alkalies from the oxygen with which they are 

 combined, and preserved them for examination ; and decom- 

 posed other bodies formerly supposed to be simple. 



It was in reasoning upon this general hypothesis likewise, 

 that I was led to the discovery which is the subject of this 

 paper. 



Copper is a metal only weakly positive in the electro-che- 

 mical scale; and, according to my ideas, it could only act 



upon 



