294 SCIENCE OF COMMON THINGS. 



Principles and processes of electro-metallurgy. 



melted / quartz, sulphur, magnesia, slate, and lime are 

 liquefied ; and the diamond fuses, boils, and becomes 

 converted into coal. 



19 OG What is ekctrotyping, or electro-metallurgy ? 



It is the art or process of depositing, from a metallic 

 solution, through the agency of galvanic electricity, a 

 coating or film of metal upon some other substance. 



19O7 Upon what principles is it accomplished f 



The process is based on the fact, that when a galva- 

 nic current is passed through a solution of some metal, 

 as a solution of sulphate of copper (sulphuric acid and 

 copper), decomposition takes place / the metal is sepa- 

 rated in a metallic state, and attaches itself to the nega- 

 tive pole, or to any substance that may be attached to 

 the negative pole; while the acid or other substance 

 before in combination with the metal, goes to, and is 

 deposited on the positive pole. 



In this way a medal, a wood-engraving, or a plaster cast, if attached 

 to the negative pole, may be covered with a coating of copper; if the 

 solution had been one containing silver or gold, the substance would 

 have been covered with a coating of silver or gold instead of copper. 



19 OS How can the thickness of the deposits be regulated f 



The thickness of the deposit, providing the supply 

 of the metallic solution be kept constant, will depend 

 on the length of time the object is exposed to the influ- 

 ence of the 'battery. 



In this way, a coating of gold thinner than the thinnest gold-leaf can 

 be laid on, or it may be made several inches or feet in thickness, if 

 desired. 



The process of electrotyping has been strikingly taken advantage of in 

 reproducing expensive engraved plates, as the map-plates of the Coast 

 Survey of the United States. The plate of the map, usually on copper, is 

 frequently the work of years under the hand of the engraver, the cost 

 being counted by thousands of dollars. If the plate, when finished, were 

 printed on directly, the pressure of the paper a few hundred times would 

 goon obliterate the faint lines of the engraving on the metal, and the plate 

 would soon become injured or spoiled. But now the original plate is 

 uever printed on. but an electrotype on copper is taken from it, at a very 

 email expense ; and this may be repeated almost indefinitely, thus afford- 

 mg fresh plates for printing whenever required. 



