il," of specific gravity 1.04. Where the nature of
the coals distilled for gas is such that the tar contains too little
anthracene to be economically recovered, the creosote-oil fraction is
carried right to the end; but otherwise, that is in most cases, a last
fraction is made at about the temperature 270 deg. C., above which the
"anthracene oil" or "green oil" is obtained up to the finish of the
distillation.

[Illustration: FIG. 3.--Condensing Worm (side elevation).]

During the light-oil period the firing must be performed very
cautiously, especially where the water has not been well removed, to
prevent bumping and boiling over. It has been observed that, apart
from the water, those tars incline most to boiling over which contain
an unusual quantity of "fixed carbon." During this period cold water
must be kept running through the cooler. The distillate at once
separates into water (gas-liquor) and light oil, floating at the top.
Towards the end of this fraction the distillation seems to cease, in
spite of increasing the fires, and a rattling noise is heard in the
still. This is caused by the combined water splitting off from the
bases and phenols and causing slight explosions in the tar.

As soon as the specific gravity approaches 1.0, the supply of cold
water to the cooler is at least partly cut off, so that the
temperature of the water rises up to 40 deg. C. This is necessary because
otherwise some naphthalene would crystallize out and plug up the
pipes. If a little steam is injected into the still during this period
no stoppage of the pipes need be feared in any case, but this must be
done cautiously.

When the carbolic oil has passed over and the temperature in the still
has risen to about 240 deg. C., the distillate can be run freely by always
keeping the temperature in the cooler at least up to 40 deg. C. The
"creosote oil" which now comes over often separates a good deal of
solid naphthalene on cooling.

The last fraction is made, either when the thermometer indicates 270 deg.
C., or when "green grease" appears in the distillate, or simply by
judging from the quantity of the distillate. What comes over now is
the "anthracene oil." The firing may cease towards the end as the
steam (with the vacuum) will finish the work by itself. The water in
the cooler should now approach the boiling-point.

The point of finishing the distillation