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. [Illustration: FIG. 4.--DETAILS OF AN INDUSTRIAL VOLTAMETER.] The constants of the voltameter established by Commandant Renard are as follows: Height of external electrode 3.405 m. " internal " 3.290 " Diameter of external " 0.300 " " internal " 0.174 " The iron plate employed is 2 millimeters in thickness. The electric resistance is about 0.0075 ohm. The apparatus gives 365 amperes under 2.7 volts, and consequently nearly 1 kilowatt. Its production in hydrogen is 158 liters per hour. It is clear that, in an industrial exploitation, a dynamo working under 3 volts is never employed. In order to properly utilize the power of the dynamo, several voltameters will be put in series--a dozen, for example, if the generating machine is in proximity to the apparatus, or a larger number if the voltameters are actuated by a dynamo situated at a distance, say in the vicinity of a waterfall. Fig. 3 will give an idea of a plant for the electrolysis of water. It remains for us to say a few words as to the net cost of the hydrogen and oxygen gases produced by the process that we have just described. We may estimate the value of a voltameter at a hundred francs. If the apparatus operates without appreciable wear, the amortizement should be calculated at a very low figure, say 10 per cent., which is large. In continuous operation it would produce more than 1,500 cubic meters of gas a year, say a little less than one centime per cubic meter. The caustic soda is constantly recuperated and is never destroyed. The sole product that disappears is the distilled water. Now one cubic meter of water produces more than 2,000 cubic meters of gas. The expense in water, then, does not amount to a centime per cubic meter. The great factor of the expense resides in the electric energy. The cost of surveillance will be minimum and the general expenses _ad libitum_.