GALVANISM. 415 



trotype coating of copper deposited upoa them, and from the copper plates 

 BO formed the book was printed. The great advantage of this is, that tho 

 copper being harder than the ordinary type metal, is more durable, and re- 

 sists the wear of printing from its surface lor a longer period. 



„ , ^ The improvement eS'ected by electro-metallurerv in enerav- 



How has the . . ^ .^, ■' , . ■, j 



electrotype mg IS very great. \\ hen a copper plate is engraved, and im- 



process affected pressions printed off from it, oulv the first few, called '"proof 

 engraving" ^ ^ ' • '^ 



impressions," possess the fineness of the engraver's deUneation. 



The plate rapidly wears and becomes deteriorated. But by the electrotyp© 

 process, the original plate can at once be multiplied into a great many plates 

 as good as itselij and an unlimited number of the finest impressions pro- 

 cured. 



In this way the map plates of the Coast Survey of the United States, some 

 of which require the labor of the engraver for years, and cost thousands of 

 dollars, are reproduced — the original plate being never printed from. 



One of the simplest illustrations of metallic deposit by electro-chemical ao- 

 tion is afforded by the foUowing experiment : — Put a piece of silver in a glass 

 containing a solution of sulphate of copper, and into the same glass insert a 

 piece of zina Xo change will take place in either metal so long as they are kept 

 apart ; but as soon as they touch, the copper will be deposited upon the sil- 

 ver, and if it be allowed to remain, the part immersed will be completely 

 covered with copper, which will adhere so firmly that mere rubbing alone will 

 not remove it. 



Hoir does the 785. When two metals which are positive 

 m'^tTis °*^aff'-'^? and negative in their electrical relations to 

 their durabuity? ^^^^ other, are brought in contact, a galvanic 

 action takes place which promotes chemical change in the 

 positive metal, but opposes it in the negative metal. 



Thus, when sheets of zinc and copper immersed in dilute 

 What are lUn?- .,,,,. j- .. j tu» 



trations of this acid touch each other, the zinc oxydizes or rusts more, and the 



principle ? copper less rapidly, than without contact. Iron nails, if used 



in fastening copper sheathing to vessels, nist much quicker than when in other 

 situations, not in contact with the copper. The reason is, that the contact of 

 the two metals excites galvanic action, which causes the iron to rust speedily, 

 but protects the copper. 



What is pii- What is called galvanized iron, is iron cov- 

 vanizediron? ^^^^^ eutirclv, or in part, with a coating of 

 zinc. The galvanic action between the two oxydizes the 

 zinc, but protects the iron from rust. 



Copper, when immersed in sea-water, rapidly wastes by the 

 j[t"J^pt to priT- chemical action of the oxygen dissolved in sea- water, but if 

 tect the sheath- j^ \^q brought in contact with zinc, or some metal that is more 

 *i«McorrosilJn? electro-positive than itself the zinc will undergo a rapid 



