generally said that for the purpose of tar-distillers the tar
is all the more valuable the less other materials than real coal have
been used by the gas-maker. All these materials--bog-head shale,
bituminous lignite and so forth--by destructive distillation yield more
or less paraffinoid oils, which render the purification of the benzols
very difficult and sometimes nearly impossible for the purposes of the
manufacturer of coal-tar colours.

Neither too high nor too low a temperature should have been observed in
gas-making in order to obtain a good quality of tar. Since in recent
times most gas retorts have been provided with heating arrangements
based on the production of gaseous fuel from coke, which produce higher
temperatures than direct firing and have proved a great economy in the
process of gas-making itself, the tar has become of decidedly inferior
quality for the purposes of the tar-distillers, and in particular yields
much less benzol than formerly.

Entirely different from gas-tar is the tar obtained as a by-product from
those (Scottish) blast furnaces which are worked with splint-coal. This
tar contains very little aromatic hydrocarbons, and the phenols are of
quite a different character from those obtained in the working of
gas-tar. The same holds good of oil-gas tars and similar substances.
These should not be worked up like gas-tars.

The ordinary yield of tar in the manufacture of coal-gas is between 4
and 5% of the weight of the coal. Rather more is obtained when passing
the gas through the apparatus of E. Pelouze and P. Audouin, where it is
exposed to several shocks against solid surfaces, or by carrying on the
process at the lowest possible temperature, as proposed by H. J. Davis,
but this "carbonizing process" can only pay under special circumstances,
and is probably no longer in practical use.

All coal-tars have a specific gravity above that of water, in most cases
between 1.12 and 1.20, but exceptionally up to 1.25. The heavier tars
contain less benzol than the lighter tars, and more "fixed carbon,"
which remains behind when the tars are exhausted of benzol and is a
decidedly objectionable constituent. All tars also mechanically retain a
certain quantity of water (or rather gas-liquor), say, 4% on the
average, which is very obnoxious during the process of distillation, as
it leads to "bumping," and therefore ought to be previously removed by
prolonged settling, preferably at a slightly elevate