The pumping now ceases, and the temperature of the retorts is raised to about 800° C. The workman is able to judge the temperature with sufficient accuracy by means of the small inspection holes, H, fitted with panes of mica, through which the color of the heat in the furnace can be distinguished. The pumps are now reversed and the process of exhaustion begins. At Westminster the pressure in the retorts is reduced to about 1½ in. of mercury. In this partial vacuum the oxygen is given off rapidly, and if forced by the pumps through another pipe and away into an ordinary gas holder, where it is stored for use. With powerful pumps such as are used in the plant under notice the whole of the oxygen can be drawn off in an hour, and from one charge a yield of about 2,000 cub. ft. is obtained. With a less perfect vacuum the time is longer - even as much as four hours. The whole operation of charging and exhausting the retorts can be completed in from three to four hours. As soon as the evolution of oxygen is finished, the doors, K, and ventilators, L, may be opened and the retorts cooled for recharging.
The cost of producing oxygen at Westminster, under specially expensive conditions, is high - about 12s. per 1,000 cub. ft. When we consider, however, that the cost should only embrace attendance, fuel, wear and tear, and a little lime and soda for the purifiers, that the consumption of fuel is small, the wear and tear light, and that the raw material - air - is obtained for nothing, it ought to be possible to produce the gas for a third or fourth of this amount in most of our great manufacturing centers, where the price of fuel is but a third of that demanded in London, and where provision could be made for economizing the waste heat, which is entirely lost in the Westminster installation. Moreover, in estimating this cost all the charges are thrown on the oxygen; were there any means of utilizing the 4,000 cub. ft. of nitrogen at present blown away as waste for every thousand cubic feet of oxygen produced, the nitrogen would of course bear its share of the cost.
The question of the application of the oxygen is one which must be determined in its manifold bearings mainly by the experiments of chemists and scientific men engaged in industrial work. Having ascertained the method by which and the limit of cost within which it is possible to use oxygen in their work, it can be seen whether by Brin's process the gas can be obtained within that limit.
Mr. S.R. Ogden, the manager of the corporation gasworks at Blackburn, has already made interesting experiments on the application of oxygen in the manufacture of illuminating gas. In order to purify coal gas from compounds of sulphur, it is passed through purifiers charged with layers of oxide of iron. When the oxide of iron has absorbed as much sulphur as it can combine with, it is described as "foul." It is then discharged and spread out in the open air, when, under the influence of the atmospheric oxygen, it is rapidly decomposed, the sulphur is separated out in the free state, and oxide of iron is reformed ready for use again in the purifiers. This process is called revivification, and it is repeated until the accumulation of sulphur in the oxide is so great (45 to 55 per cent.) that it can be profitably sold to the vitriol maker. Hawkins discovered that by introducing about 3 per cent. of air into the gas before passing it through the purifiers, the oxygen of the air introduced set free the sulphur from the iron as fast as it was absorbed.
Thus the process of revivification could be carried on in the purifiers themselves simultaneously with the absorption of the sulphur impurities in the gas.
A great saving of labor was thus effected, and also an economy in the use of the iron oxide, which in this way could be left in the purifiers until charged with 75 per cent. of sulphur. Unfortunately it was found that this introduction of air for the sake of its oxygen meant also the introduction of much useless nitrogen, which materially reduced the illuminating power of the gas. To restore this illuminating power the gas had to be recarbureted, and this again meant cost in labor and material. Now, Mr. Ogden has found by a series of conclusive experiments made during a period of seventy-eight days upon a quantity of about 4,000,000 cub. ft. of gas, that by introducing 1 per cent. of oxygen into the gas instead of 3 per cent. of air, not only is the revivification in situ effected more satisfactorily than with air, but at the same time the illuminating power of the gas, so far from being decreased, is actually increased by one candle unit.
So satisfied is he with his results that he has recommended the corporation to erect a plant for the production of oxygen at the Blackburn gas works, by which he estimates that the saving to the town on the year's make of gas will be something like £2,500. The practical observations of Mr. Ogden are being followed up by a series of exhaustive experiments by Mr. Valon, A.M. Inst. C.E., also a gas engineer. The make of an entire works at Westgate is being treated by him with oxygen. Mr. Valon has not yet published his report, as the experiments are not quite complete; but we understand that his results are even more satisfactory than those obtained at Blackburn.
In conclusion we may indicate a few other of the numerous possible applications of cheap oxygen which might be realized in the near future. The greatest illuminating effect from a given bulk of gas is obtained by mixing it with the requisite proportion of oxygen, and holding in the flame of the burning mixture a piece of some solid infusible and non-volatile substance, such as lime. This becomes heated to whiteness, and emits an intense light know as the Drummond light, used already for special purposes of illumination. By supplying oxygen in pipes laid by the side of the ordinary gas mains, it would be possible to fix small Drummond lights in place of the gas burners now used in houses; this would greatly reduce the consumption of gas and increase the light obtained, or even render possible the employment of cheap non-illuminating combustible gases other than coal gas for the purpose.
Two obstacles at present lie in the way of this consummation - the cost of the oxygen and the want of a convenient and completely refractory material to take the place of the lime. Messrs. Brin believe they have overcome the first obstacle, and are addressing themselves, we believe, to the removal of the second. Again, the intense heat which the combustion of carbon in cheap oxygen will place at the disposal of the metallurgist cannot fail to play an important part in his operations. There are many processes, too, of metal refining which ought to be facilitated by the use of the gas. Then the production of pure metallic oxides for the manufacture of paints, the bleaching of oils and fats, the reduction of refractory ores of the precious metals on a large scale, the conversion of iron into steel, and numberless other processes familiar to the specialists whose walk is in the byways of applied chemistry, should all profit by the employment of this energetic agent. Doubtless, too, the investigation into methods of producing the compounds of nitrogen so indispensable as plant foods, and for which we are now dependent on the supplies of the mineral world, may be stimulated by the fact that there is available by Brin's process a cheap and inexhaustible supply of pure nitrogen. - Industries.