100 INFLUENCE OF GALVANISM IN PROTECTING 



manner I have little doubt that iron would be protected in sea- 

 water." 1 



INFLUENCE OF GALVANISM IN PROTECTING METALS FROM DESTRUCTION 

 BY OXIDATION AND SOLUTION. 



The galvanic influence of one metal in protecting another is in 

 relation to their negative and positive qualities together with their 

 conducting powers (pp. 28-9). Their relations in sea-water are 

 silver, copper, bismuth, tin, lead, cobalt, nickel, iron, cadmium, zinc; 

 the first the most negative, the last the most positive in the series. 

 So that, according to this scale, the further apart the metals may be 

 which are selected for experiment, the more decided will be the 

 power of the positive to protect the negative. Copper and zinc 

 operate more strongly together than iron and zinc. 



A metal that is insoluble when placed singly in a fluid, may be 

 made soluble by connection with a relatively negative metal placed 

 in the same fluid. For example, pure zinc put into muriatic acid is 

 unaffected, but when connected with copper in the same fluid it is 

 rapidly operated upon. Or a metal may be soluble in a fluid alone, 

 but may be rendered insoluble by connection with a relatively 

 positive metal, which undergoes decomposition instead. Thus: 

 copper is dissolved in sea-water when alone, but when a piece of 

 zinc is connected with it, the copper is unaffected. This last effect 

 is the substance of Davy's method of protection alluded to by Dr. 

 Faraday, in applying the principle of which it is necessary to take 

 into consideration 



1st, The amount and power of electricity generated by the 

 connected metals in the same fluid; and 



2d, The conducting power of the metal which is being pro- 

 tected. 



1st. The amount and power of the electricity evolved is in pro- 

 portion to the difference of the relative negative and positive condi- 

 tions of the metals employed. The more negative the coated metal 

 is, the less it requires protection, although its powers of protection 

 .are the greatest. And the more positive the coated metal, the more 

 liable it is to be destroyed, and the greater the amount of electri- 

 city required to protect it; but unfortunately it is less able to 

 generate this electricity when in contact with another metal. Thus 

 these two conditions are opposed to the application of galvanic 

 influence for protecting iron. 



Suppose, for example, that 4 square inches of zinc, in connection 

 with 4 square feet of copper, give out sufficient electricity to protect 



1 See Practical Mechanics' and Engineers' Magazine, 1 st series, Vol*. iv. page 326. 

 Glasgow, 1845. 



