ir 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.

913. 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 (986.). 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 resisting 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
affinities opposing decomposition are in this way diminished, and the
elements of the water can then be separated by a current of lower
intensity.

914. Advantage may be taken of this principle to interpolate more minute
degrees into the scale of initial intensities already referred to (909.
911.) 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.

* * * * *

915. Returning to the consideration of the source of electricity (878.
&c.), 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 is only another mode of the exertion
of chemical forces. It is, the production of the _electric spark_ before
any contact of metals is made, and by the exertion of _pure and unmixed
chemical forces_. The experiment, which will be descr