. They are usually made of
cast iron. The still itself is either an upright or a horizontal
cylinder, heated by a steam-coil, of a capacity of from 1000 to 2000
gallons. The superposed columns contain from 20 to 50 compartments of
a width of 2-1/2 or 3 ft. The vapours pass into a cooler, and from this
the distillate runs through an apparatus, where the liquor can be seen
and tested, into the receivers. The latter are so arranged that the
water passing over at the same time is automatically removed. This is
especially necessary, because the last fraction is distilled by means
of pure steam.

The fractions made in the steam distillation vary at different works.
In some places the pure hydrocarbons are net extracted and here only
the articles called: "90 per cent. benzol," "50 per cent. benzol,"
"solvent naphtha," "burning naphtha" are made, or any other commercial
articles as they are ordered. The expression "per cent." in this case
does not signify the percentage of real benzene, but that portion
which distills over up to the temperature of 100 deg. C., when a certain
quantity of the article is heated in glass retorts of a definite
shape, with the thermometer inserted in the liquid itself. By the
application of well-constructed rectifying-columns and with proper
care it is, however, possible to obtain in this operation nearly pure
benzene, toluene, xylene, and cumene (in the two last cases a mixture
of the various isomeric hydrocarbons). These hydrocarbons contain only
a slight proportion of thiophene and its isomers, which can be removed
only by a treatment with fuming sulphuric acid, but this is only
exceptionally done.

Sometimes the _pyridine bases_ are recovered from the tarry acid which
is obtained in the treatment of the light oil with sulphuric acid, and
which contains from 10 to 30% of bases, chiefly pyridine and its
homologues with a little aniline, together with resinous substances.
The latter are best removed by a partial precipitation with ammonia,
either in the shape of gas or of concentrated ammoniacal liquor. This
reagent is added until the acid reaction has just disappeared and a
faint smell of pyridine is perceived. The mixture is allowed to
settle, and it then separates into two layers. The upper layer,
containing the impurities, is run off; the lower layer, containing the
sulphates of ammonia and of the pyridine bases, is treated