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electrical and chemical changes. 
It is always to be understood that strong or concentrated 
solutions of acids and alkalies are employed; for in cases 
where the quantity of acid or alkaline 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 
positive, 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-sulphu- 
ret 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 
detailed in the last section. 
When, in electrical combinations containing one metal, 
water, or a neutro-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- 
