Page:All the Year Round - Series 2 - Volume 1.djvu/70

 nothing seems stranger than that this heavy, black, tarry liquid should produce oil as pure as water, and solid paraffine as white as marble. And yet the marvel is wrought daily, and on a scale which supplies distant markets of the world with oil. It is a mere question of refining. The black liquor is, as it were, boiled, washed, and bleached, re-boiled, re-washed, and re-bleached, until the last particle of its darkness and impurity is purged away. The first step in the work of refinement is in some respects similar to the previous process of decomposition. The crude tarry liquid is put into stills, which we may call huge boilers of gigantic strength, with movable doors or lids. When the stills have been filled, the doors are closed, and the joints are stuffed with clay, so as to render the interior perfectly air-tight. Fires are then lighted in the furnaces below the boilers, and kept up to a steady heat, till the fluid inside distils over and is transmuted again into vapour. This vapour, as in the former instance, permeates through another series of condensing pipes, and, during its transit, is retransmuted into liquor, and flows into a second reservoir. Collected in this tank, the oil shows abundant evidence of the severity of the ordeal through which it has been put. It passed into the stills black, and of the consistency of treacle; it has come out of a dark green colour, and of the consistency of pea-soup. A large portion of the coal-black has, in fact, been boiled out of it, which is now to be found in the bottom of the boilers in the shape of a lustrous compact residue resembling coke, for which it makes a very good substitute.

The next stage in the process of purification is of a different character. The dark green liquor is transferred to tanks, and a certain quantity of strong sulphuric acid is added. The acid is employed in order still further to bleach the oil, and purge it of some more of the impurity with which it is so largely impregnated. To effect this object it is essential that the oil and the acid should be mixed up or assimilated as much as possible—a work of some difficulty, on account of the tendency of the former to float on the top, by reason of its lighter specific gravity. This tendency is neutralised by the action of a revolving stirrer fitted with blades, which, when put in motion, beats and agitates the two liquids, and causes them to mingle equally. For four hours is this operation continued, until, under the biting influence of the acid, the dark green oil changes to pale green, and gives token of having parted with much of the grosser substances that had rendered it dull and opaque. The stirrers being at length stopped, the liquor is allowed to settle, and the organic impurities that have been separated from it by the action of the vitriol, collect in the bottoms of the tanks. The lees in this case assume the shape of a coarse acid tar, which is also used as a substitute for fuel.

The oil, thus far cleansed of its foulness, is now transferred to clean tanks, mixed with a strong solution of caustic soda, and again subjected to the beating of the stirrers. The action of the alkali extracts a good deal more of the colouring matter, and changes the pale green to yellow. At the end of a second period of four hours the liquor is allowed to settle, is drawn off from the lees as before, is pumped into the stills and re-distilled, and is again brought back to be put through the acid and alkali bleaching process; the result being its assumption of a clear, pale, yellow colour. When in this stage of its preparation the oil contains the elements of no less than four different products, each valuable as articles of commerce, to separate which is the next care of the manufacturer.

The separation is effected merely by distilling the oil at various temperatures. At the lowest temperature the lightest and most volatile parts of the oil pass off in the shape of vapour. Upon being cooled, by passing through pipes, this vapour yields a liquid which, upon being distilled by itself, gives a light, transparent, inflammable fluid known by the name of naphtha, the specific gravity of which is considerably less than that of the naphtha derived from coal-tar. This naphtha is largely employed as a substitute for turpentine in india-rubber works, where it is employed to dissolve the materials used in that branch of manufacture. At the temperature next to the lowest, those parts of the oil that are next to naphtha in point of volatility are taken off, distilled and condensed, and yield paraffine or lamp oil. The processes of purification and distillation are repeated with this oil till it has assumed the requisite degree of purity, and becomes transparent and almost free from smell. A gallon of this oil weighs, about eight and a quarter pounds, and is, in point of illuminating power, nearly equal to one gallon and a quarter of American petroleum. A yet higher temperature than that which is necessary for the production of the burning oil produces a thick, heavy, lubricating oil, used in vast quantities in the Lancashire factories for oiling the machinery, and also by watch and clock and philosophical instrument makers. This oil, when it comes from the still, is largely impregnated with solid paraffine, and when it cools it assumes the consistency of grease, the paraffine having coagulated into crystals. Before the lubricating oil can be made available for what it is intended, these crystals must be separated from it; and here again another operation, but one of a very simple nature, is requisite. The oil is poured into thick canvas bags, which are placed in hydraulic presses. Pressure is then applied with such force that the oil is squeezed out of the bags, leaving the crystals within. The oil thus squeezed out is the lubricating oil, and is ready for the market; the crystals are the paraffine in embryo which has so often been admired in the shape of candles.

When turned out of the bags the paraffine is in its coarsest state, and is of a dirty yellow colour. This hue is the result of the quantity of oily matter which the substance, in spite of its frequent purgings, still retains. Its perfect and final purification is effected by the repetition of a single process, continued till the requisite clearness is obtained. The paraffine is dissolved in heated naphtha, and is kept in