urement. Water, therefore, acidulated by sulphuric acid, is the
substance I shall generally refer to, although it may become expedient in
peculiar cases or forms of experiment to use other bodies (843.).

707. The first precaution needful in the construction of the instrument was
to avoid the recombination of the evolved gases, an effect which the
positive electrode has been found so capable of producing (571.). For this
purpose various forms of decomposing apparatus were used. The first
consisted of straight tubes, each containing a plate and wire of platina
soldered together by gold, and fixed hermetically in the glass at the
closed extremity of the tube (Plate V. fig. 60.). The tubes were about
eight inches long, 0.7 of an inch in diameter, and graduated. The platina
plates were about an inch long, as wide as the tubes would permit, and
adjusted as near to the mouths of the tubes as was consistent with the safe
collection of the gases evolved. In certain cases, where it was required to
evolve the elements upon as small a surface as possible, the metallic
extremity, instead of being a plate, consisted of the wire bent into the
form of a ring (fig. 61.). When these tubes were used as measurers, they
were filled with the dilute sulphuric acid, inverted in a basin of the same
liquid (fig. 62.), and placed in an inclined position, with their mouths
near to each other, that as little decomposing matter should intervene as
possible; and also, in such a direction that the platina plates should be
in vertical planes (720).

708. Another form of apparatus is that delineated (fig. 63.). The tube is
bent in the middle; one end is closed; in that end is fixed a wire and
plate, _a_, proceeding so far downwards, that, when in the position
figured, it shall be as near to the angle as possible, consistently with
the collection, at the closed extremity of the tube, of all the gas evolved
against it. The plane of this plate is also perpendicular (720.). The other
metallic termination, _b_, is introduced at the time decomposition is to be
effected, being brought as near the angle as possible, without causing any
gas to pass from it towards the closed end of the instrument. The gas
evolved against it is allowed to escape.

709. The third form of apparatus contains both electrodes in the same tube;
the transmission, therefore, of the electricity, and the consequent
decomposition, is far more rapid than in the separate tubes. The resulting