Renard
naturally directed his attention to the production of pure hydrogen.
The solutions of the question are, in fact, alike in principle, and
yet they have been developed in a very different manner, and we
believe that Commandant Renard's process is the completest from an
industrial standpoint. We shall give an account of it from a
communication made by this eminent military engineer, some time ago,
to the French Society of Physics.

_Transformations of the Voltameter._--In a laboratory, it is of no
consequence whether a liter of hydrogen costs a centime or a franc. So
long as it is a question of a few liters, one may, at his ease, waste
his energy and employ costly substances.

The internal resistance of a voltameter and the cost of platinum
electrodes of a few grammes should not arrest the physicist in an
experiment; but, in a production on a large scale, it is necessary to
decrease the resistance of the liquid column to as great a degree as
possible--that is to say, to increase its section and diminish its
thickness. The first condition leads to a suppression of the platinum,
and the second necessitates the use of new principles in the
construction of the voltameter. A laboratory voltameter consists
either of a U-shaped tube or of a trough in which the electrodes are
covered by bell glasses (Fig. 1, A and B). In either case, the
electric current must follow a tortuous and narrow path, in order to
pass from one electrode to the other, while, if the electrodes be left
entirely free in the bath, the gases, rising in a spreading form, will
mix at a certain height. It is necessary to separate them by a
partition (Fig. 1, C). If this is isolating and impermeable, there
will be no interest in raising the electrodes sensibly above its lower
edge. Now, the nearer together the electrodes are, the more it is
necessary to lower the partition. The extension of the electrodes and
the bringing of them together is the knotty part of the question. This
will be shown by a very simple calculation.

[Illustration: FIG. 1.--A, B, COMMONEST FORMS OF LABORATORY
VOLTAMETERS. C, DIAGRAM SHOWING ASCENT OF BUBBLES IN A VOLTAMETER.]

The visible electrolysis of water begins at an E.M.F. of about 1.7 V.
Below this there is no disengagement of bubbles. If the E.M.F. be
increased at the terminals of the voltameter, the current (and
consequently the production of gas) will become proportional to the
excess of the value over 1.7 V; but, at t