ft for
decision by future examination.

702. In any investigation of these points, great care must be taken to
exclude water; for if present, secondary effects are so frequently produced
as often seemingly to indicate an electro-decomposition of substances, when
no true result of the kind has occurred (742, &c.).

703. It is evident that all the cases in which decomposition _does not
occur, may_ depend upon the want of conduction (677. 413.); but that does
not at all lessen the interest excited by seeing the great difference of
effect due to a change, not in the nature of the elements, but merely in
their proportions; especially in any attempt which may be made to elucidate
and expound the beautiful theory put forth by Sir Humphry Davy[A], and
illustrated by Berzelius and other eminent philosophers, that ordinary
chemical affinity is a mere result of the electrical attractions of the
particles of matter.

[A] Philosophical Transactions, 1807, pp. 32, 39; also 1826, pp. 387,
389.

P v. _On a new measure of Volta-electricity._

704. I have already said, when engaged in reducing common and voltaic
electricity to one standard of measurement (377.), and again when
introducing my theory of electro-chemical decomposition (504. 505. 510.),
that the chemical decomposing action of a current _is constant for a
constant quantity of electricity_, notwithstanding the greatest variations
in its sources, in its intensity, in the size of the _electrodes_ used, in
the nature of the conductors (or non-conductors (307.)) through which it is
passed, or in other circumstances. The conclusive proofs of the truth of
these statements shall be given almost immediately (783, &c.).

705. I endeavoured upon this law to construct an instrument which should
measure out the electricity passing through it, and which, being interposed
in the course of the current used in any particular experiment, should
serve at pleasure, either as a _comparative standard_ of effect, or as a
_positive measurer_ of this subtile agent.

706. There is no substance better fitted, under ordinary circumstances, to
be the indicating body in such an instrument than water; for it is
decomposed with facility when rendered a better conductor by the addition
of acids or salts; its elements may in numerous cases be obtained and
collected without any embarrassment from secondary action, and, being
gaseous, they are in the best physical condition for separation and
meas