DEPOSITION OF METALS UPON ONE ANOTHER. 



COATING OF IRON WITH COPPER. 



BESIDES making articles of solid copper, we may at a small cost give 

 a coating of copper to another metal, such as iron, which if kept in 

 a dry place will retain the appearance of copper for any length of 

 time. But in covering iron with copper, or any one metal with 

 another, great care must be taken that a proper kind of solution 

 be used. 



It is a familiar fact, that if a piece of iron, such as the blade of a 

 knife, be dipped into a solution of sulphate of copper, it receives a 

 coating of that metal. This is often described as the result of 

 galvanic action, but there is no more galvanic action in this than in 

 any ordinary chemical combination ; it is simply a case of chemical 

 substitution : the acid that is in union with the copper having a 

 stronger attraction for iron, leaves the copper, and combines with 

 the iron : the copper is left on the surface of the iron, but the two 

 metals not having sufficient polar attraction to cause them to adhere 

 so firmly as to exclude the action of the acid, the copper is under- 

 mined, and falls to the bottom of the solution as a powder. After 

 some copper has fallen upon the surface of the iron, local galvanic 

 action is induced between it and the iron ; but this secondary action 

 is altogether distinct from that which first takes place. 



Any solution that has the power to give a metallic coating to a 

 metal when dipped into it, should not be used to coat that metal by 

 electricity. 



The attraction of the common mineral acids for the ordinary 

 metals is as follows : Zinc, iron, copper, nickel, silver, gold, 

 platinum. 



If the metal to be deposited be copper held in solution by an 

 acid, say sulphuric acid, then iron or zinc cannot be coated with 

 copper from this solution ; the acid having a greater attraction for 

 these metals, will leave the copper and combine with them as 

 described above ; but if the metal to be coated be any of those 

 under copper, in the above table, then no chemical action will take 



