DEPENDENCE OF THE CURRENT ON CHEMICAL ACTION. 
121 
2048. A piece of rusty iron in the same solution is powerfully negative. So also 
is a platinum plate with a coat of protoxide, or peroxide, or native carbonate of iron 
on it (2045.). 
2049. This result is one of those effects which has to be guarded against in the 
experiments formerly described (1826. 1886.). If what appears to be a clean plate 
of iron is put into a dilute solution of the sulphuret of potassium, it is first negative 
to platinum, then neutral, and at last generally feebly positive ; if it be put into a 
strong solution, it is first negative, and then becomes neutral, continuing so. It cannot 
be cleansed so perfectly with sand-paper, but that when immersed it will be negative, 
but the more recently and well the plate has been cleansed, the shorter time does this 
state continue. This effect is due to the instantaneous oxidation of the surface of 
the iron during its momentary exposure to the atmosphere, and the after reduction 
of this oxide by the solution. Nor can this be considered an unnatural result to 
those who consider the characters of iron. Pure iron in the form of a sponge takes 
fire spontaneously in the air ; and a plate recently cleansed if dipped into water, or 
breathed upon, or only exposed to the atmosphere, produces an instant smell of 
hydrogen. The thin film of oxide which can form during a momentary exposure is, 
therefore, quite enough to account for the electric current produced. 
2050. As a further proof of the truth of these explanations, I placed a plate of iron 
under the surface of a solution of the sulphuret of potassium, and rubbed it there with 
a piece of wood which had been soaking for some time in the same sulphuret. The 
iron was then neutral or very slightly positive to platinum connected with it. Whilst 
in connection with the platinum it was again rubbed with the wood so as to acquire a 
fresh surface of contact; it did not become negative, but continued in the least degree 
positive, showing that the former negative current was only a temporary result of 
the coat of oxide which the iron had acquired in the air. 
2051. Nickel appears to be subject to the same action as iron, though in a much 
slighter degree. All the circumstances were parallel, and the proof applied to iron 
(2050.) was applied to it also, with the same result. 
2052. So all these phenomena with protoxides and peroxides agree in referring the 
current produced to chemical action ; not merely by showing that the current de- 
pends upon the action, but also that the direction of the current depends upon the 
direction which the chemical affinity determines the exciting or electromotive anion 
to take. And it is, I think, a most striking circumstance, that these bodies, which 
when they can and do act chemically produce currents, have not the least power 
of the kind when mere contact only is allowed (1869.), though they are excellent 
conductors of electricity, and can readily carry the currents formed by other and 
more effectual means. 
2053. With such a mass of evidence for the efficacy and sufficiency of chemical 
MDCCCXL. r 
