terminations _p, p, p_ rested on the litmus, and the other three _n, n, n_
on the turmeric paper. On working the machine for a short time only, acid
was evolved at _all_ the poles or terminations _p, p, p_, by which the
electricity entered the solution, and alkali at the other poles _n, n, n_,
by which the electricity left the solution.

322. In all experiments of electro-chemical decomposition by the common
machine and moistened papers (316.), it is necessary to be aware of and to
avoid the following important source of error. If a spark passes over
moistened litmus and turmeric paper, the litmus paper (provided it be
delicate and not too alkaline,) is reddened by it; and if several sparks
are passed, it becomes powerfully reddened. If the electricity pass a
little way from the wire over the surface of the moistened paper, before it
finds mass and moisture enough to conduct it, then the reddening extends as
far as the ramifications. If similar ramifications occur at the termination
_n_, on the turmeric paper, they _prevent_ the occurrence of the red spot
due to the alkali, which would otherwise collect there: sparks or
ramifications from the points _n_ will also redden litmus paper. If paper
moistened by a solution of iodide of potassium (which is an admirably
delicate test of electro-chemical action,) be exposed to the sparks or
ramifications, or even a feeble stream of electricity through the air from
either the point _p_ or _n_, iodine will be immediately evolved.

323. These effects must not be confounded with those due to the true
electro-chemical powers of common electricity, and must be carefully
avoided when the latter are to be observed. No sparks should be passed,
therefore, in any part of the current, nor any increase of intensity
allowed, by which the electricity may be induced to pass between the
platina wires and the moistened papers, otherwise than by conduction; for
if it burst through the air, the effect referred to above (322.) ensues.

324. The effect itself is due to the formation of nitric acid by the
combination of the oxygen and nitrogen of the air, and is, in fact, only a
delicate repetition of Cavendish's beautiful experiment. The acid so
formed, though small in quantity, is in a high state of concentration as to
water, and produces the consequent effects of reddening the litmus paper;
or preventing the exhibition of alkali on the turmeric paper; or, by acting
on the iodide of potassium, evolvi