arrangement. We cannot, in fact, expect that the entire piping shall
be always in such conditions that no difference in pressure can occur.
The levels are brought back to equality within the effective limits by
interposing between the voltameter and the piping an apparatus called
a compensator, which consists of two vessels that communicate in the
interior part through a large tube. The gases enter each vessel
through a pipe that debouches beneath the level of the water. If a
momentary stoppage occurs in one of the conduits, the water changes
level in the compensator, but the pressure remains constant at the
orifice of the tubes. The compensator is, as may be seen, nothing more
than a double Mariotte flask. When it is desired to obtain pure gases,
there is introduced into the compensator a solution of tartaric acid,
which retains the traces of alkalies carried along by the current of
gas. The alkaline solution, moreover, destroys the ozone at the moment
of its formation.

It will be seen that laboratory studies have furnished all the
elements of a problem which is now capable of entering the domain of
practice. The cheapness of the raw materials permits of constructing
apparatus whose dimensions will no longer be limited except by reasons
of another nature. The electrodes may be placed in proximity at will,
owing to the use of the porous partition. It may be seen, then, that
the apparatus will have a considerable useful effect without its being
necessary to waste the electric energy beyond measure.

_Industrial Apparatus._--We have shown how the very concise researches
of Commandant Renard have fixed the best conditions for the
construction of an industrial voltameter. It remains for us to
describe this voltameter itself, and to show the rendering of it.