664 TREATMENT OF BATTERY. 



can be moved ; the top of the carbon is then placed within the bind- 

 ing screw, the nnt m lifted by the thmnb and middle finger of the 

 left hand nntil b is in close contact with the horizontal portion of a, 

 and pressing a firmly down upon the carbon by means of the two 

 last fingers of the left hand, the pressure screw is screwed tightly np 

 with the right hand. 



The onter vessel, the zinc cylinder, and the porons vessel are like 

 the same parts in Grove's element, bnt there is no cover npon the 

 porons cell. 



Zinc, as we know, is dissolved by dilnte snlphuric acid. "While 

 the circnit is closed, and a galvanic current passes, solution of zinc 

 goes on, whatever the kind of element nsed, and consequently 

 the zinc is gradually consumed and must be renewed. But the con- 

 sumption of zinc may be prevented while the circuit is open, and the 

 total consumption therefore considerably lessened, if the zinc is 

 amalgamated, that is, covered with a superficial layer of an alloy of 

 zinc and mercury. The zinc cylinders obtained from the dealers 

 when purchasing galvanic elements are usually amalgamated ; but 

 if the student has to amalgamate zinc, he should dip it for a 

 short time in dilute sulphuric acid, and then, holding it over a 

 capacious vessel (a basin or pan), a few drops of mercury are 

 poured upon the metal. If the acid has produced a clean surface 

 upon the zinc, the mercury will spread itself out upon it at once ; 

 otherwise it is rubbed well everywhere over the surface, inside and 

 outside the cylinder, by means of a little piece of cloth moistened 

 by the dilute acid. The mercury left in the basin after the opera- 

 tion is .alloyed with zinc, and should therefore not be poured back 

 into a bottle containing pure mercury. Freshly amalgamated zinc 

 is uniformly bright and silver white ; on standing for a long time in 

 the air it gets dull and grey, and a few small drops of mercury 

 collect on the surface. If such grey zinc is placed again in dilute 

 acid, the mercury diffuses itself again all over the surface, and the 

 silvery colour is restored, but not the former brightness. 



Of the effect of amalgamating zinc the student can easily convince 

 himself by putting two thin strips of zinc, 3 cm long and l cm wide, 

 one amalgamated, and the other not, into test tubes half filled with 

 dilute sulphuric acid. The unamalgamated strip dissolves rapidly 

 with brisk evolution of hydrogen, and finally disappears, while the 

 amalgamated strip shows only here and there a few bubbles of gas, 

 and remains almost intact for an indefinite time. 



Amalgamated zinc which has become grey causes also effervescence 

 at first, when placed in the acid, but this goes on only until the 

 zinc has become white again. 



