232 THE ELECTRIC TELEGRAPH. 



metallic impurities, after a time, collect on its surface, and 

 form an insoluble crust, which lessens the electro-motive force 

 of the pair by preventing the dissolution of the zinc and by 

 its different electric condition ; while amalgamated zinc, on 

 the contrary, separates the particles from its surface, and 

 allows them to fall off to the bottom. 



The way in which zinc plates or cylinders are amal- 

 gamated is by dipping them first, for a minute, into a vessel 

 containing dilute muriatic acid, and then by plunging them 

 into a bath of metallic mercury. After remaining here for 

 a minute, they are taken out and thrown into a tub of 

 clean water, where the superfluous mercury is allowed to 

 drain off. 



Berjot, in order to save the quantity of mercury which 

 this method entails, has suggested a process of amalga- 

 mating without metallic mercury. He dips the zincs into 

 a solution of mercury in nitro-muriatic acid for a few 

 seconds only, to completely amalgamate them. He recom- 

 mends the process as easy, cheap, and certain.* 



II. MEASUREMENT OF THE GALVANIC CURRENT. 



14. The Voltameter. When two platinum wires con- 

 tinuations of the poles of a galvanic battery are plunged 

 into water, bubbles of gas are observed to form on each 

 of them, rising to the surface when their bulk becomes 

 sufficiently great to overcome their adhesion to the metal. 

 These gases are hydrogen and oxygen the former is 

 developed at the zinc electrode, and the latter at the copper. 



The various forms of apparatus constructed for collecting 

 and measuring the volumes of these gases are called volta- 

 meters. One of the handiest for measuring large quan- 

 tities of gas developed in stated intervals is the following : 



* The solution is made by dissolving 200 grammes of mercury, at a mode- 

 rate heat, in 1 kilogramme of nitro-muriatic acid (1 part NHO d to 3 parts 

 H 01.), and after complete solution, by the addition of another kilogramme of 

 nitro-muriatic acid. This quantity of solution is said to be sufficient for 

 amalgamating from 150 to 200 zincs. 



