ng iodine.

325. By moistening a very small slip of litmus paper in solution of caustic
potassa, and then passing the electric spark over its length in the air, I
gradually neutralized 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.

326. 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.

327. I have already had occasion to refer to an experiment (265. 309.) 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 ordinary electricity. By covering fine wires with glass or
other insulating substances, and then removing only so much matter as to
expose the point, or a section of the wires, and by passing electricity
through two such wires, the guarded points of which were immersed in water,
Wollaston found that the water could be decomposed even by the current from
the machine, without sparks, and that two streams of gas arose from the
points, exactly resembling, in appearance, those produced by voltaic
electricity, and, like the latter, giving a mixture of oxygen and hydrogen
gases. But Dr. Wollaston himself points out that the effect is different
from that of the voltaic pile, inasmuch as both oxygen and hydrogen are
evolved from _each_ pole; he calls it "a very close _imitation_ of the
galvanic phenomena," but adds that "in fact the resemblance is not
complete," and does not trust to it to establish the principles correctly
laid down in his paper.

328. This experiment is neither more nor less than a repetition, in a
refined manner, of that made by Dr. Pearson in 1797[A], and previously by
MM. Paets Van Troostwyk and Deiman in 1789 or earlier. That the experiment
should never be quoted as proving true electro-chemical decomposition, is
sufficiently evident from the circumstance, that the _law_ which regulates
the transference and final place of the evolved bodies (278. 309.) has no
influence here. The water is decomposed at both poles independently of each
other, and the oxygen and hydrogen evolved at the wires are the elements of
the water existing the instant before in those places. That the