i 6	Faraday's Researches
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 eleo
' tricity through the air from either the point p or n, iodine will
be immediately evolved.
 59.	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 (58)
ensues.
 60.	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, evolving iodine.
6r. By moistening a very small slip of litmus paper in solu-
tion of caustic potassa, and then passing the electric spark over
its length in the air, I gradually neutralised the alkali, and
ultimately rendered the paper red; on drying it, I found that
nitrate of potassa had resulted from the operation, and that the
paper had become touch paper.
 62.	Either litmus  paper or white paper,  moistened in a
strong solution of iodide of potassium, offers therefore a very
simple, beautiful, and ready means of illustrating Cavendish's
experiment of the formation of nitric acid from the atmosphere.
 63.	I have already had occasion to refer to an experiment
(i, 45) made by Dr. Wollaston, which is insisted upon too
much, both by those who oppose and those who agree with the
accuracy of his views respecting the identity of voltaic and