"The permanence of the baryta is mainly dependent on its physical condition, the use of reduced pressure during deoxidation, and con sequent avoidance of excessively high tempera tures, and the careful purification of the air used. It was found possible to dispense with change of temperature in the reaction, change of pressure being alone trusted to for deter about 1 I. H. P. per 1,000 feet of oxygen pro duced per day, the ratio decreasing for larger plants.
"It is necessary that the barium oxide should be as hard and as porous as possible, and this is best obtained by preparing it by igniting the nitrate. The nitrate fuses and decomposi tion soon commences with evolution of a mix ture of oxygen and oxides of nitrogen. This action continues for about two to three hours, during which time the contents of the crucible remain in ebullition. A porous mass is then left, which is heated for another hour to com plete, as far as possible, the decomposition. In this way a very hard but also very porous baryta is obtained.
"This process was thoroughly practical and economical, and large numbers of plants were erected all over the world and worked success fully for many years. Some plants are still working, but in the last few years the process has been largely superseded by the still cheaper liquid air process, which also produces oxygen of greater purity." Commercial Manufacture.—The commercial manufacture of oxygen has thus reached the stage where oxygen in any appreciable amount is made either by the electrolytic method or by air liquefaction.
By Electrolysis.— When an electric current is passed through water containing sulphuric acid or an alkali (to facilitate the passage of the current) hydrogen is formed at the cathode and oxygen at the anode. By providing suit able means for collecting the gases, they are obtained in a very pure condition.
This method lends itself readily to the prep aration of hydrogen and oxygen for the laboratory or for larger industrial uses.
With each volume of oxygen two volumes of hydrogen are evolved.
For laboratory purposes the electrolyte may be dilute sulphuric acid and the electrodes of platinum.
For the commercial production of these gases the electrolytic cell is made as compact as possible; the electrolyte is a solution of caustic potash or caustic soda, and the elec trodes are of cast iron (the anode being nickelplated to increase efficiency and to pre vent rusting). A very efficient type of cell
now on the market is shown in the accompany ing cuts.
Essentially this consists of a thin rectangu lar box to which are bolted two cast-tron plates or electrodes. The cavity thus formed between the electrodes is divided by a dia phragm of asbestos fabric. In the upper part of the cast-iron frame are reservoirs for the electrolyte from which it is fed to the two sides of the diaphragm. The two gas cham bers which permit the separation of the gas from moisture and electrolyte are also located in the upper part of the frame of the cell, and serve as gas traps and gas offtakes, as well as automatic pressure controlling devices. At the bottom of the frame are the communicating passageways which permit equalization of densities in the electrolyte. The inside of the electrodes carry a great number of pyramid shaped projections which greatly increase the area in contact with the electrolyte and which facilitate the release of the gases at the gen erating surface.
The interior of the cell, both sides of the asbestos diaphragm, is filled with a solution of caustic potash or caustic soda in distilled water. When the electric current is passed through, the gases form and rise upward along the surfaces of the electrodes to the collecting chambers above. Any tendency for them to flow from one electrode to the other is pre vented by the intervening diaphragm of asbestos.
In the special type of cell here described, with a normal current of 600 amperes at 22 volts and an electrolyte of caustic soda solu tion, a kilowatt-hour efficiency of 3.65 cubic feet of oxygen and 7.3 cubic feet of hydrogen is obtained. It has a normal rated capacity of 4.8 cubic feet of oxygen and 9.6 cubic feet of hydrogen per clock hour. This cell can be operated at current strengths up to 1,000 am peres, at which point the capacity is greatly increased though the efficiency is somewhat lower. The gases are extremely pure, the oxygen analyzing as high as 99.7 per cent and the hydrogen as high as 99.9 per cent.