To this inconvenience of porous vessels is added their fragility, their high price, and the impossibility of obtaining them of the dimensions that large apparatus would call for.  The selection of asbestos cloth is therefore clearly indicated; but, as it does not entirely separate the gases, except at a pressure that does not exceed a few centimeters of water, it was always necessary to bring back the variation of the level to these narrow limits by a special 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.

[Illustration:  Fig. 3.—­Plant for the industrial electrolysis of water.]

The industrial voltameter consists of a large iron cylinder.  A battery of such voltameters is shown to the left of Fig. 3, and one of the apparatus, isolated, is represented in Fig. 4.  The interior electrode is placed in an asbestos cloth bag, which is closed below and tied at its upper part.  It is provided with apertures which permit of the ascent of the gases in the interior of the cylinder.  The apparatus is hermetically sealed at the top, the two electrodes being naturally insulated with rubber.  Above the level of the liquid the interior electrode is continuous and forms a channel for the gas.  The hydrogen and oxygen, escaping through the upper orifices, flow to the compensator.  The apparatus is provided with an emptying cock or a cock for filling with distilled water, coming from a reservoir situated above the apparatus.