Electrolytic Intensity 183

in the order in which I have placed them, those which are first
being decomposed by the current of lowest intensity. These
currents were always from a single pair of plates, and may be
considered as elementary voltaic forces.

Iodide of potassium (solution).

Chloride of silver (fused).

Protochloride of tin (fused).

Chloride of lead (fused).

Iodide of lead (fused).

Muriatic acid (solution).

Water; acidulated with sulphuric acid.

648. It is essential that, in all endeavours to obtain the
relative electrolytic intensity necessary for the decomposition of
different bodies, attention should be paid to the nature of the
electrodes and the other bodies present which may favour
secondary actions (721). If in electro-decomposition one of the
elements separated has an affinity for the electrode, or for
bodies present in the surrounding fluid, then the affinity resist-
ing decomposition is in part balanced by such power, and the
true place of the electrolyte in a table of the above kind is
not obtained: thus, chlorine combines with a positive platina
electrode freely, but iodine scarcely at all, and therefore I
believe it is that the fused chlorides stand first in the preceding
table. Again, if in the decomposition of water not merely
sulphuric but also a little nitric acid be present, then the water
is more freely decomposed, for the hydrogen at the cathode is
not ultimately expelled, but finds oxygen in the nitric acid, with
which it can combine to produce a secondary result; the affini-
ties opposing decomposition are in this way diminished, and
the elements of the water can then be separated by a current
of lower intensity.

649. Advantage may be taken of this principle to interpolate
more minute degrees into the scale of initial intensities already
referred to (644, 646) than is there spoken of; for by combining
the force of a current constant in its intensity, with the use of
electrodes consisting of matter, having more or less affinity for
the elements evolved from the decomposing electrolyte, various
intermediate degrees may be obtained.

650. Returning to the consideration of the source of electricity
(613, etc.),, there is another proof of the most perfect'kind that
metallic contact has nothing to do with the -production ^of
electricity in the voltaic circuit, and further, that electricitye (644), would probably include