PRINTING 317 



294. How Pure Metal is obtained from Ore. When ore is 

 mined, it contains in addition to the desired metal many other 

 substances. In order to separate out the desired metal, the 

 ore is placed in some suitable acid bath, and is connected with 

 the positive terminal of a battery, thus taking the place of 

 the silver slab in the last Section. When current flows, 

 any pure metal which is present is dissolved out of the ore 

 and is deposited on a convenient negative electrode, while 

 the impurities remain in the ore or drop as sediment to the 

 bottom of the vessel. Metals separated from the ore by 

 electricity are called electrolytic metals and are the purest 

 obtainable. 



295. Printing. The ability of the electric current to de- 

 compose a liquid and to deposit a metal constituent has prac- 

 tically revolutionized the process of printing. Formerly, type 

 was arranged and retained in position until the required num- 

 ber of impressions had been made, the type meanwhile being 

 unavailable for other uses. Moreover, the printing of a sec- 

 ond edition necessitated practically as great labor as did the 

 first edition, the type being necessarily set afresh. Now, how- 

 ever, the type is set up and a mold of it is taken in wax. This 

 mold is coated with graphite to make it a conductor and is 

 then suspended in a bath of copper sulphate, side by side 

 with a slab of pure copper. Current is sent through the 

 solution as described in Section 293, until a thin coating of 

 copper has been deposited on the mold. The mold is then 

 taken from the bath, and the wax is replaced by some metal 

 which gives strength and support to the thin copper plate. 

 From this copper plate, which is an exact reproduction of the 

 original type, many thousand copies can be printed. The 

 plate can be preserved and used from time to time for later 

 editions, and the original type can be put back into the cases 

 and used again. 



