198 Sir H. Davy on Elecirical and Chemical Changes. 



plied as to suffer a certain solution of the copper*; and an 

 absolute remedy for adhesions, is to be sought for by other 

 more refined means of protection, and which appear to be in ■• 

 dicated by these researches. 



The nails used in ships are an alloy of copper and tin, which 

 I find is slightly negative with respect to copper, and it is on 

 these nails that the first adhesions uniformly take place : a 

 slightly positive and slightly decomposable alloy would pro- 

 bably pi-event this effect, and I have made some experiments 

 favourable to the idea. 



In general, all changes in metals which would indicate the 

 power of chemical attraction, are easily determined by elec- 

 trical means. Thus I found copper hardened by hammering 

 negative to rolled copper; copper (to use the technical language 

 of manufacturers) both overpoled and underpoled, containing 

 in one case probably a little charcoal and in the other a little 

 oxide, negative to pure copper. A specimen of brittle copper, 

 put into my hands by Mr. Vivian, but in which no impurity 

 could be detected, was negative with respect to soft copper. 



In general, very minute quantities of the oxidable metals 

 render the alloy positive, unless it becomes harder, in which 

 case it is generally negative. As I have mentioned before, 

 amalgams of the oxidable metals are usually positive, not only 

 to mercury, but even to the pure metals. 



There are probably few chemical opei'ations which electrical 

 changes do not influence, and either increase or modify. In 

 the rusting of iron, for instance, the oxide formed by the con- 

 tact of moisture becomes the negative surface, and exalts the 

 oxidability of the mass of metallic iron, and the rust conse- 

 quently extends in a circle. 



The precipitations of metals have been already traced to 

 causes of this kind, and many metallic solutions must belong 

 to the same order of phenomena. 



I have pointed out in former papers some of the cases of 

 electro-chemical protection, which I have no doubt, when the 

 principles are well understood, will be generally adopted ; and 

 others are constantly occurring. I shall mention one, — the 

 preservation of the iron boilers of steam engines by mtrodu- 



• A common cause of adhesions of weeds or shell fish, is the oxide of 

 iron formed and deposited round the protectors. In the only experiment 

 in which zinc has been employed for this purpose in actual service, the 

 ship returned after two voyages to the West Indies, and one to Quebec, 

 perfectly clean. 



The exi)eriment was made by Mr. Lawrence, of Lombard-street, who in 

 his letter to me states that the rudder, whicii was not protected, had cor- 

 roded in the usual manner. 



cing 



