The generators work without intervention of manual labour, and they are arranged side by side in a room having a constant temperature ; the gases produced are collected in iron piping and, after passing through a regulator, enter the gasometers. The attendance is limited to refilling each of the generators with three to four litres of water per day. In respect of safety it is advisable frequently to analyse the gases, easily done by Hempel's apparatus or by Bassani's electric density meter, or still better by an aerostatic scale, which has the advantage of giving continuous indications.

In reference to the commercial point it is well to remember that one coulomb liberates 0.0829 milligramme of oxygen and 0.703 milligramme of hydrogen, or 0.058 cubic centimetres of oxygen and 0.116 cubic centimetres of hydrogen at a temperature of 0° C. and at a pressure of 760 millimetres, i.e., atmospheric pressure.

One ampere-hour liberates 208.8 cubic centimetres of oxygen and 417.6 cubic centimetres of hydrogen under the same conditions in respect of temperature and pressure. As the operation generally takes place at a temperature of 15° to 20° C, it may be admitted, for practical purposes, and taking account of all the losses, that one ampere-hour will give 0.40 litres of hydrogen and 0.20 litres of oxygen.

As to the electro-motive force required,

One gramme of water releases, in its formation, about 3.8 calorics, or 3.8 X 425 = 1.615 kilogramme metres.

One coulomb decomposes 0.0000933 gramme of water; in order to decompose one gramme of water an expenditure in work is therefore required represented by 1/0.0000933 C X X volts, or, expressed in kilogramme metres, 1 x X / 0.0000933 x 9.81 or, as above stated, X / 0.0000933 x 9.31 = 1.615, thus X - 1.488, or in round figures 1.5 volts. Gerard has found, however, that the electro-motive force required by a Garuti apparatus made of steel, with an electrolyte of caustic potash as pure as possible, amounts to 1.988 volts. But for industrial purposes it is not possible to go as low as that; with an electrolyte composed of caustic soda an electro-motive force of 2.4 volts is required, while with potash it may be reduced to 2.2 volts.

Assume, however, a voltage of 2.4 at the most, and a production of 0.40 litre of hydrogen and 0.20 litre of oxygen per ampere-hour, the production per kilowatt-hour amounts to - Hydrogen, 166.6 litres, equal to 37 gallons, Oxygen, 83.3 ; „ „ 18.5 „ or 250 litres = 55.5 gallons of explosive gas.

The production of one cubic metre of explosive gas (666 litres hydrogen and 333 litres oxygen) requires thus 4 kilowatts.

One cubic metre of hydrogen alone requires 6 kilowatts.

One „ „ ,, oxygen „ „ 12

Assuming a minimum voltage of 2.2, the production per kilowatt-hour would be -

Hydrogen . .. 181.8 litres Oxygen... 90.9 ,,

The production of one cubic metre explosive gas requires 3.667 kilowatts, one cubic metre of hydrogen alone 5.50 kilowatts, and one cubic metre of oxygen alone 11.00 kilowatts. Finally, the gases produced are practically pure ; the hydrogen contains an appreciable quantity of other gases, while the oxygen contains 1/2 to 4 per cent. of hydrogen, which can easily be removed.

The Production of Oxygen from Liquid Air. - It has already been mentioned on page 7 that oxygen is produced through the liquefaction of atmospheric air, principally by Linde, Knudsen, and Claude.

Linde has, as the pioneer, by his strenuous efforts and scientific demonstrations created a new industry, which becomes more and more important by the extension of the practical applications of its products, oxygen and nitrogen. The patents for the United Kingdom have been acquired by The British Oxygen Company, Limited, which have works in London, Manchester and Birmingham producing oxygen of any required purity. The Linde system is simple and continuous; the plant may be erected almost anywhere without restrictions, requiring but a small space.

The Knudsen patents for the United Kingdom are owned by The Liquid Air Power and Automobile Company of Great Britain, Limited, having a 250 h.p. plant working at Battersea, London.

The Claude system is being worked principally in France, represented in the United Kingdom by The British Liquid Air Company, Limited.

As to the cost of production of oxygen by the various systems, it is difficult to give exact figures, but the following statements have been obtained so far as the cost of the machines is concerned, in addition to which must be considered the spaces required, maintenance of plant, and other more or less important points bearing upon the question.

Assuming a production of 250 cubic metres of oxygen per twenty-four hours, the cost of the plants actually required for the production of the oxygen, but without cost of erection and of the compressors required for the compression of the gas, has been given as follows : -

Brin's process .

£2,900

Garuti's electrolytic process

4,000

Linde's system ...

2,350

The cost per cubic metre is given by Linde's system at 12.7 pfg.; Garuti's system at 8 pfg.

When the annual consumption exceeds 5,000 cubic metres, or when the reduction in price would increase its application to welding and cutting, then it would be advisable to produce oxygen at the place of operation. Oxygen is a gas which can easily be produced at any place in similar manner as ordinary coal gas. By so doing its price would be materially reduced.

The oxygen used for welding must be free from chlorine, while its usual mixture with 5 per cent. of nitrogen and 2 to 3 per cent. of hydrogen is of no importance.