fig. 44, so
that the part _d, f_ shall be nearly upright, whilst the whole is resting
on the three bearing points _p, e, f_ place these as in fig. 43; the points
_p, n_ then become the decomposing poles. In this way surfaces of contact,
as minute as possible, can be obtained at pleasure, and the connexion can
be broken or renewed in a moment, and the substances acted upon examined
with the utmost facility.

313. A coarse line was made on the glass with solution of sulphate of
copper, and the terminations _p_ and _n_ put into it; the foil _a_ was
connected with the positive conductor of the machine by wire and wet
string, so that no sparks passed: twenty turns of the machine caused the
precipitation of so much copper on the end _n_, that it looked like copper
wire; no apparent change took place at _p_.

314. A mixture of equal parts of muriatic acid and water was rendered deep
blue by sulphate of indigo, and a large drop put on the glass, fig. 43, so
that _p_ and _n_ were immersed at opposite sides: a single turn of the
machine showed bleaching effects round _p_, from evolved chlorine. After
twenty revolutions no effect of the kind was visible at _n_, but so much
chlorine had been set free at _p_, that when the drop was stirred the whole
became colourless.

315. A drop of solution of iodide of potassium mingled with starch was put
into the same position at _p_ and _n_; on turning the machine, iodine was
evolved at _p_, but not at _n_.

316. A still further improvement in this form of apparatus consists in
wetting a piece of filtering paper in the solution to be experimented on,
and placing that under the points _p_ and _n_, on the glass: the paper
retains the substance evolved at the point of evolution, by its whiteness
renders any change of colour visible, and allows of the point of contact
between it and the decomposing wires being contracted to the utmost degree.
A piece of paper moistened in the solution of iodide of potassium and
starch, or of the iodide alone, with certain precautions (322.), is a most
admirable test of electro-chemical action; and when thus placed and acted
upon by the electric current, will show iodine evolved at _p_ by only half
a turn of the machine. With these adjustments and the use of iodide of
potassium on paper, chemical action is sometimes a more delicate test of
electrical currents than the galvanometer (273.). Such cases occur when the
bodies traversed by the current are bad conducto