404 Sir Humphry Davy on the relations of 
saline solutions, which when they contain oxygen or common 
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 strong- 
est 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 gas, which is probably oxygen, on the platinum 
in contact with the alkali ; and that the acid tends to circulate 
towards the negative surface, and the alkali towards the 
positive, is shown by the circumstance of the rapid neutrali- 
zation of the two menstrua, though separated by asbestus 
moistened in distilled water. 
VI. Of combinations consisting of two conductors of the more 
perfect class , and one fluid. 
The order in which metallic bodies exhibit electricities on 
contact, as is well known, is intimately connected with their 
relative oxidability, the most oxidable metal being positive 
