There are two distinct systems of producing: illuminating gas by the retorting of oil. The direet cracking and gasifying in iron retorts is the plan in general use, and fractional distillation in peculiar iron vessels is a recent development.

The first of the above methods is exemplified in the well-known Pintsch's oil-gas process, which is also extensively used in the lighting of railway-carriages. The carbonizing plant for the supply of 150 lights is shown in Fig. 663, and consists of a bench of four retorts in two tiers, arranged so that each upper retort is connected to the lower one beneath it, by a short pipe, and the two ovens, thus coupled, form one complete retort for the vaporization of oil. Only one "couple" of retorts is used at a time, the other being a stand-by for use during repairs, &c

Fig. 663 -Ground-floor Plan of Works for pintsch's oil - gas Process.

The retorts are from six-and-a-half to seven feet in length, and are supported by cross-walls over a furnace of ordinary type. They vary in shape, but are mostly of section, 10 inches wide and 8 inches in height They are heated by direct firing from the furnace l»elow, the lower retort having a temperature somewhat in excess of the upper one, for the specific purpose of completing the gasification of the oil. Each "couple" has a separate and distinct hydraulic seal.

The oil to be vaporized is run into the top retort through a tube having two bends, forming a natural seal, and technically known as a "gooseneck" seal, as shown in Fig. 664. It will readily be seen that a sufficient head of oil must be decanted into the seal to cause the liquid to flow into the retort against the pressure from within. As the resulting gas has to force its way through a hydraulic seal, condenser, and purifiers, and into the gasholder, this pressure is frequently sufficient to support a column of oil seven or eight inches high. Under such conditions it would be necessary to obtain a level of liquid some seven indies higher in the longer limb of the U-tube, to ensure the free passage of the oil into the retort.

The oil flowing into the upper retort is conducted for about three feet of its length in a wrought-iron trough, permitting it to receive a little heat before coming into contact with the cast-iron retort, lest the latter should fracture on account of the sudden extremes of temperature. In this retort the more volatile constituents of the oil are immediately converted into vapour, and pass with the heavier hydrocarbons into the lower retort The latter performs two functions. The remaining heavy hydrocarbons arc gasified, and the whole of the vapours and crude gases are subjected to the reflected heat of this retort, which tends to resolve the mixture into less complex bodies and compounds of more stable character. Such action is technically known as "fixing", or superheating. So that in explaining the use of the two retorts, we may say generally, that the upper retort "cracks" or vaporizes the oil, and the lower retort fixes the vapours into illuminating hydrocarbon gases.

The manufacture of oil-gas is carried on with very little labour or supervision. Having adjusted the flow of oil to the temperature of the retorts, the production of gas, in many plants on this system, continues without appreciable intermission for a period of six months. The life of the retorts is rather less than those used for the distillation of coal. The scale or carbon deposit, left in oil retorts, is more difficult to remove, and at times appears to be fused into the iron as a form of slag. The "life" of a retort, however, may safely be put at six months, and each retort can be detached and replaced without difficulty and at small expense.

The gas, as it leaves the lower retort, passes through a hydraulic seal of small dimensions, kept cool by the passage of a continuous supply of water through the vessel. A large quantity of the heavy hydrocarbons is deposited at this point as tar, the liquid being carried by an overflow-pipe to the tar-well. From the seal, the gas passes to the condenser. The hydraulic seal and condenser perform similar offices to those used in coal-gas plants, the one to prevent "back pressure "into the retort, and the other to remove the last particles of conden-aible compounds.

Fig. 664- Goose. neck' Seal for Running Oil into Retort.

The condensers in oil-gas plants are known as "atmospheric condensers . being so designed as to allow the surrounding air to circulate freely between the upright pipes, or annular casing, down which the gas is conducted. By this means a constant uptake of cool air robs the gas of its high temperature, and, together with the diminished flow generally provided for in this apparatus, causes a quantity of unstable compounds to separate out as heavy oil or light tar. In large installations this liquid is frequently run off separately, as it forms a very excellent liquid fuel, and can be used in a "spray". The heavier tar is invariably burnt in the furnace under the retorts for economizing fueL The total quantity of tar thus obtained generally amounts to about one-third of the oil used.

From the condenser the gas is passed through purifiers to remove sulphur compounds, and finally into the gasholder. For the purpose of lighting railway-carriages, the gas is compressed in iron cylinders to a pressure of from 12 to 15 atmospheres, but for domestic use a pressure of 6 inches proves very suitable.

The quality of the gas is very high, being four times as rich as coal-gas made from ordinary or common coal; its illuminating power ranges from 55 to 65 candles. In consequence of this property, the amount of gas consumed is considerably diminished. Flat-flame burners for this gas are designed for a consumption of one-half to three-quarters of a cubic foot per hour, giving an illuminating power of 10 to 12 candles. Regenerative burners for the consumption of this gas are now in vogue, increasing its efficiency as an illuminant, and an able soft light of 40 candle-powers can be procured by a consumption of 2 feet per hour.

The amount of oil which can be properly treated by each couple of retorts varies with the temperature to which they are raised, a general quantity being 2½ gallons per hour. The yield of purified gas, taking a fair average, is 80 cubic feet per gallon of oil. The supply of gas for 150 lights in winter would necessitate the carl ionization of eleven gallons per diem. In practice, however, the plant would be operated twice a week, and the gasholder designed to store three or four days' consumption. A very good oil for the purpose of gasification can be procured at a cost of 2¼d. to 2½d. per gallon. The cost of this oil-gas will be about 3s. per 1000 cubic feet.

Heating and cooking stoves have been designed for the special use of oil-gas and other gases very rich in hydrocarbons, and give satisfaction in every way equal to stoves burning coal-gas.

Owing to the slight attention required, the freedom from nuisance, the diminutive size, and the low cost of the plant, this oil-gas system should obtain consideration. There are two items, which, being satisfactorily dealt with, should favour its adoption. First, the supply of fuel for the furnace, and secondly, the carriage of the oil.

A more modern method of obtaining illuminating gas from oil is the invention of Mr. Young, and is known as the Peebles process. In this process the oil is subjected to a very ingenious system of fractional distillation. A cylindrical bottle-shaped iron retort, about seven feet long, having a diameter of two feet in the body, gradually reduced to one foot at the mouth, has a spherical blank or closed end. The retort is laid with a slight fall towards the closed end, and is heated to a red or dull red heat by a furnace, or by waste gases from other sources. Oil is allowed to run into the retort in such a stream that it is vaporized before it has travelled the entire length. The lighter constituents are vaporized in the neck of the vessel; the remaining portions (consisting of the more dense hydrocarbons) How towards the end, and thus receive a prolonged exposure to the heat of the retort, which is necessary to convert them into vapour and gases. Owing to the considerable diameter of the retort, the vapours are exposed to a large amount of surface-contact and heat-reflection, and the effect of these is felt for a longer period by those heavier hydrocarbon gases generated at the end of the retort The object of this arrangement is recognized when we consider that the lighter hydrocarbons are stable compounds of high illuminating power, and the character of the heavier hydocarbons undergoes a change of an improving nature upon subjection to a superheating or "fixing" action. It is observed, therefore, that in this process the cxposure of the vapours from the more volatile constituents of the oil to the reflected heat of the retort is considerably less in comparison.

The gas thus produced passes through an oil seal to prevent "beck pressure" in the retort, travels along a pipe conveying the oil to the retort, through a "washer" or "scrubber" (where the gas, in small divided streams, is brought into intimate contact with the oil), and finally into a purifier and gasholder In its passage through seal, oil-pipe, and "scrubber", those constituents of the gas which are not of a stable character are dissolved by the oil, and by this agency returned to the retort to undergo another process of "cracking" and "fixing". In this manner, gas of a permanent description is produced. A great advantage of this system is that no tar is produced.

The practical results of plants on this system show that the whole of the oil cannot be converted into gas. A small quantity is lost in the formation of a hard brilliantly-crystalline coke, which has a great value in metallurgical manipulations, and which can always be sold for over £ I per ton. The volume of gas obtained from a gallon of oil treated by this process lies between 85 and 88 cubic feet, having an illuminating power of 60 candles. The deposit of coke is about 2 lbs. per gallon of oil.

The conditions under which the oil is treated in this system of gasification are very sciontitic, and are attended with an increased yield of gas. The adoption of this process, however, should greatly depend upon the supply of fuel for heating the retort Oil-gas by this arrangement is obtained at a cost of 2s. 7d. per 1000 cubic feet