on the Action of Water and Air on Lead. 93 



metals, particularly iron, partly because it appeared that ef- 

 fects that have been described might be modified by the con- 

 tact of other metals, and partly because the tables of the elec- 

 tric order of metals given by different philosophers differ from 

 each other in the place assigned to lead. Thus in Dr. Henry's 

 Elements we have first Sir H. Davy's table of metals, the 

 liquid conductor being acid or saline solutions, and it runs 

 thus — zinc, iron, tin, lead, copper, each being positive to 

 those that follow : some pages further in the same work we 

 find Volta's arrangement*, which runs thus — zinc, lead, tin, 

 iron, copper. This also accords with Pouillet's table. 



40. My experiments were made with a galvanometer having 

 two needles and a glass thread, as constructed by Dr. Ritchie. 

 The plates of the metals were 2 inches long by 1^ wide, and 

 retained at equal distances by fixing a slip of wood a quarter 

 of an inch thick between them. Copper wires were soldered 

 to the plates to connect them with the cups of the galvano- 

 meter. The liquids were contained in cylindrical earthen- 

 ware cups holding about 4 ounces of water. 



I found that when the liquids employed were either distilled 

 water, spring water, solutions of neutral salts, dilute sulphuric, 

 muriatic or nitric acids, lime-water, or solution of caustic 

 potash, and the metals both bright, that lead was to iron 

 constantly as the zinc to copper in the common voltaic ar- 

 rangements, or that the current was from the lead to the iron 

 through the liquid. But when the lead was tarnished before 

 it was put into the water, or when, being put into acids or 

 saline solutions, it was suffered to remain immersed a short 

 time, the deflection of the needle rapidly diminished, and 

 after a time took place in the opposite direction. 



In the Bakerian Lecture for 1826, Sir H.Davy has given an 

 example, and pointed out the general cause, of such reversals 

 in the relations of metals ; but the particular case of lead is not 

 there noticed, and it occupies the same relative place in an 

 amended table of the order of metals given in that memoir 

 which it originally held in the series published by the same 

 philosopher in the Journal of the Royal Institution published 

 in 1802. 



41. The power of deflecting the needle as measured by the 

 torsion of glass threads given by plates of lead and iron, com- 

 pared with that given by similar combinations of zinc and 

 copper with the same liquids, is shown approximately in the 



* This is given in a memoir by Volta, Ann. de Chimic, torn. xl. p. 243, 

 1802, but he speaks of it as having been previously published. 



