of Electrical and Chemical Changes. 101 



where the quantity of acid or alkaUne matter is very small 

 and the chemical action of the metals strong, there is some- 

 times a different order. Thus zinc and tin tarnish immedi- 

 ately even in a weak solution of potassa, and, so tarnished, 

 they are negative to the same metals in weak solutions of 

 muriatic or sulphuric acid; but in experiments of this kind it 

 is easy to determine the true circumstances by changing the 

 poles ; the negative side, when the energies of the alkali and 

 acid are weak, will be determined by the tarnish or coat of 

 oxide formed. 



Solutions of sulphurets act in these combinations like alkali, 

 with circumstances depending upon the formation of new 

 compounds, according to the law explained in the last section. 

 In combinations, of which the elements are hydro-sulphuret 

 and acid, the metal in the hydro-sulphuretted solution is po- 

 sitive, and that in the acid negative; but with alkalies and 

 hydro-sulphurets, and zinc and tin, the metal in the solution 

 of alkali is positive, and that in the solution of hydro-sul- 

 phuret, negative : with silver and palladium the opposite order 

 occurs, and with copper there is nearly a balance of powers, 

 or changes of power, dependent upon the circumstances de- 

 tailed in the last section. 



When, in electrical combinations containing one metal, 

 water or a neuti'o-saline solution is in one of the cups, and 

 alkali or acid in another, the result is usually such as might 

 be anticipated, — the side of the metal in the alkali is positive, 

 that in the acid negative, and that in the neutro-saline solu- 

 tion in the opposite state. There are however certain neutro- 

 saline solutions, which when they contain oxygen or com- 

 mon air, act upon the more oxidable metals, and such have a 

 power or energy of their own ; thus zinc, and tin, and copper 

 in solution of common salt, are positive to the same metals in 

 distilled water ; and the surfaces of the same metals in weak 

 muriatic acid are positive with respect to the surfaces in water 

 or saline solutions. In combinations, in which weak and 

 strong solutions of acids or of alkalies are the two fluids, both 

 being of the same kind, the electrical action is usually feeble ; 

 but the surface in the strongest alkali is most positive, and in 

 the acids the result usually depends upon the 'nature of the 

 solution ; if oxide is formed and deposited, the strongest acid 

 is negative with respect to the diluted one. 



The chemical changes produced in combinations of this 

 kind, are best observed in cases where the metals undergo no 

 change; for instance, with platinum, diluted sulphuric acid, 

 and solution of potassa. In this combination, hydrogen soon 

 appears on the platinum in the acid, and a very small quantity 



of 



