102 INFLUENCE OF GALVANISM, &C. 



local cause, negative, the other parts are thus rendered positive ; the 

 result is, that upon the borders of an individual sheet either over- 

 lapping or underlying its neighbouring sheet, an electric current is 

 excited, passing through the stratum of moisture which may inter- 

 vene, and the ultimate effect is that the positive edge is dissolved as 

 effectually as if cut by a knife. The evil arising in one place may 

 be so contagious as to affect a whole neighbourhood sometimes the 

 whole side of a ship's bottom. 



In fresh water iron cannot be protected any length of time, for the 

 zinc coating speedily passes into a blackish substance, which peels off 

 and exposes the iron to rust. When iron is simply exposed to the 

 air, a good coating of zinc is a sure protection. We have seen iron 

 of various qualities coated by the electro-process and exposed to the 

 atmosphere, in all weathers, for several years, without being more 

 affected than a piece of zinc would be. In spots where abrasion has 

 taken place by accident, the protecting power of the zinc is lost, and 

 the iron rusts as if there were no zinc present. No other result, 

 however, could be anticipated, as there can be no electric excitation 

 without a liquid to connect the two metals. 



The iron to be coated by zinc is to be cleaned and prepared in 

 the same manner as we have described for the purpose of covering 

 it with copper (page 95). 



