974 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



The first, of Webster & Young, is founded on the following 

 principle. Suppose that two different metals, for instance, iron 

 and copper, each of which singly would be chemically acted upon, 

 or oxydizcd by a liquid, say sea-Avater, are united by means of a 

 metallic connection, and both metals plunged in that liquid, then 

 only one will be oxydizcd (the iron), and the other (the copper) 

 will be perfectly protected at the expense of the first (the iron), 

 which will so much the strongei" be oxydized, and this protective 

 action of the first (the iron) will last till it is all dissolved or 

 oxydized. The cause of this phenomenon is, that agalvanic current 

 is excited by the electro-positive metal (the iron), which circulates 

 through the connection and the liquid and so protects the electro- 

 negative metal (the copper). 



Some fifty years ago, this principle was applied by Humphrey 

 Davy, to prevent the corrosion of the copper coating of vessels 

 by sea-water. The positive metal he used was zinc, of which dif- 

 ferent pieces were soldered at several places against the copper 

 lining of the vessel. It was, however, found that not only a pro- 

 tection against oxidyzation was afforded to the copper, which 

 lasted as long as there was any metalic zinc left, but that, also, 

 deposits of marine vegetation, of barnacles, etc., were formed 

 against the copper, so that the bottom became as foul as is the 

 case with wooden ships; so it was abandoned, because one of the 

 greatest advantages in coppering vessels is that the poisonous 

 green compound of copper continually forming on the surface has 

 a strong tendency to keep the ship clean. 



A few years ago, Webster & Young took out a patent to apply 

 this same principle for preventing incrustation in steam boilers. 

 They suspended in the water a rod or plate of copper, or any 

 other metal, electro-negative in comparison with the iron of the 

 boiler, and connected with it in such a way that any galvanic cur- 

 rent could 'circulate through the water, and back through the 

 metallic connection. They intended that the thus suspended plate, 

 by the galvanic attraction, should receive all the deposits and 

 salts from the water, which it must necessarily do, like as the 

 copper coating of ships receives ocean deposits. But as this 

 deposit on ships is formecLat the expense of the zinc plates, v.-hich 

 are soon corroded and destroyed, so, in the steam boilers, these 

 deposits are also received by the copper at the expense of the 

 boiler, which by necessity soon must be corroded and destroyed. 



Quite recently (March 12, 1867) a patent Avas granted to Parry, 



