SCIENCE AND PRACTICE. 231 



cost of the potassium was found, however, to be an objection 

 to its employment in practical telegraphy, and he conse- 

 quently used, in some experiments made at Greenwich in 

 1860, a highly electro-positive alloy of sodium and zinc in 

 porous pots in the earth. 



The system never had, and probably never will have, 

 a chance of being employed on a line of any length. It 

 costs more than a single ordinary battery, is not so easy 

 to control, and its electro-motive force can never be 

 increased beyond that of a single element ; whereas both 

 the Morse and the pointer telegraphs require a current 

 of considerable intensity to set them in motion. 



The sole advantage to be gained by its employment 

 would be its constancy, if very large plates were buried 

 deep enough in the earth to ensure them being uniformly 

 moist ; the current would last until the whole of the electro- 

 positive metal became converted into oxide a process 

 which would take a very long time. 



An economical element is arranged by burying a copper 

 plate in the damp earth, connected to an insulated wire, for 

 the positive pole, and a wire connected to gas or water 

 pipes for a negative. An iron-copper pair is thus obtained, 

 which will continue active as long as the pipes last. 



13. Amalgamated Zinc. Sturgeon* it was who suggested 

 the amalgamating of the zinc plates of galvanic elements. 

 Two important advantages were proposed and obtained by 

 it : first, amalgamated zinc is not soluble in dilute sulphuric 

 acid when no galvanic current passes from the metal to 

 the liquid, and then only to an amount which is exactly 

 equivalent to the strength of the passing current : there- 

 fore, when the circuit is open the zinc is not wasted, as is 

 the case when unamalgamated zinc is placed in acidulated 

 water ; and, secondly, amalgamated zinc is considerably more 

 electro-positive than unamalgamated. To these advantages 

 may be added that the zinc of commerce contains always 

 metallic impurities, amongst which iron, lead, cadmium, and 

 manganese are the foremost. When unamalgamated, these 

 * Researches, 1830. 



