COATING OF IRON WITH ZINC. 99 



hardly be handled without breaking, from its crystalline character. 

 When the deposition upon black lead is attempted, the best method 

 is to have the solution saturated with the salt, employing a battery 

 of 6 or 7 pairs of plates, and keeping the article on which the 

 deposit is taking place constantly in motion. 



The use of cyanide of zinc has been recommended, but for what 

 good reason it is hard to know. It is unnecessary, and its use pre- 

 sents great practical difficulties. The positive electrode becomes 

 coated, after a few minutes' working, with a white pasty matter 

 which prevents further action and stops the current. Some of 

 this white coating collected, washed, and dried in the air, gave by 

 analysis, 



Oxide of zinc 51.3 



Cyanagen 1.7 



Iron trace 



Potash 2.3 



Carbonic acid 27.8 



Matter insoluble in HC1. . . 2.5 



Water 14.8 



100.4 



The zinc is converted into carbonate of zinc; the potash is combined 

 with the cyanagen as cyanide of potassium. 



Use of Zinc Coating. The principal application of zinc is upon 

 iron, to protect it from corrosion, which it does, not only as a coat- 

 ing, but, from its more electro-positive character, it protects it by a 

 galvanic influence. The voltaic influence of zinc for protecting iron 

 is a subject that has occupied the attention of practical men for a 

 long time : it is one of high importance ; nevertheless there seems 

 yet a great deficiency in our knowledge of the extent of this influ- 

 ence, and how and when it is effective. 



Upon this subject, Professor Faraday, in the Eeport of the Har- 

 bours of Kefiige Commissioners states, "Zinced iron would no doubt 

 resist the action of sea-water so long as the surface was covered 

 with zinc, or even when partially denuded of that metal; but zinc 

 dissolves rapidly in sea-water, and after it is gone the iron would 

 follow." 



" As to voltaic protection, it has often struck me that the cast- 

 iron piles proposed for lighthouses, or beacons, might be protected 

 by zinc in the same manner as Davy proposed to protect copper 

 by iron ; but there is no doubt the corrosion of the zinc would 

 be rapid. If not found too expensive, the object would be 

 to apply the zinc protectors in a place where they could be 

 examined often, and replaced when rendered ineffective. In this 



