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 re-
transmuted 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 solution