[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.

Let us take the case in which the energy has to be borrowed from a steam engine.  Supposing very small losses in the dynamo and piping, we may count upon a production of one cubic meter of hydrogen and 500 cubic decimeters of oxygen for 10 horse-power taken upon the main shaft, say an expenditure of 10 kilogrammes of coal or of about 25 centimes—­a little more in Paris, and less in coal districts.  If, consequently, we fix the price of the cubic meter of gas at 50 centimes, we shall preserve a sufficient margin.  In localities where a natural motive power is at our disposal, this estimate will have to be greatly reduced.  We may, therefore, expect to see hydrogen and oxygen take an important place in ordinary usages.  From the standpoint alone of preservation of fuel, that is to say, of potential energy upon the earth, this new conquest of electricity is very pleasing.  Waterfalls furnish utilizable energy in every locality, and, in the future, will perhaps console our great-grandchildren for the unsparing waste that we are making of coal.—­La Nature.