244 VOLTAIC ELECTRICITY. 



386. Amalgamating the Zinc. Ordinary com- 

 mercial zinc is far from being pure. Chemically pure zinc 

 is expensive. When impure zinc is used, small closed cir- 

 cuits are formed between the particles of foreign matter 

 and the particles of zinc. This local action rapidly destroys 

 the zinc plate and contributes nothing to the general cur- 

 rent. This waste, which would not occur if perfectly pure 

 zinc were used, is prevented by frequently amalgamating 

 the zinc. This is done by cleaning the plate in dilute 

 acid and then rubbing it with mercury. 



(a.) The method of amalgamating battery zincs practised by the 

 author is as follows : In a glass vessel placed in hot water, dissolve 

 15 cu. cm. of mercury in a mixture of 170 cu. cm. of strong nitric 

 acid and 625 cu. cm. of chlorhydric (muriatic) acid. When the 

 mercury is dissolved, add 830 cu. cm. of chlorhydric acid. When 

 the liquid has cooled, immerse the battery zinc in it for a few 

 minutes, remove and rinse thoroughly with water. The liquid may 

 be used over and over until the mercury is exhausted. The quan- 

 tity here mentioned will suffice for 200 ordinary zincs or more. 

 Keep the liquid, when not in use, in a glass-stoppered bottle. 



387. Thermal Effects of Voltaic Electric- 

 ity. When a strong current is passed over a very thin 

 wire made of a poor conductor, as platinum or even iron, 

 the resistance develops heat which may render the 

 wire incandescent or even fuse or vaporize it. Thus 

 the Voltaic current is often used in firing mines in mili- 

 tary operations and blasting. (See 389.) 



(a.) If stout copper wires from the two plates of a potassium 

 bi-chromate battery (Fig. 187) have their free ends united by a very 

 fine iron wire, the passage of the current will heat it sufficiently to 

 ignite gun cotton. All known metals, even iridium and platinum, 

 have been melted in similar manner, while carbon rods have been 

 heated by a battery of 600 Bunsen's elements until they softened 

 enough for welding. 



