COAL-TAR AND WATER-GAS TAR CREOSOTES. 11 
temperature of the gases, and this in turn is governed by the tem- 
perature of the retort and the time of contact of the gases with the 
heated walls. The time of contact of the gases with the walls is 
governed to some extent by the type of retort or oven used and the 
manner in which it is filled. 
APPARATUS USED IN THE PRODUCTION OF COAL TAR. 
As indicated before, high-temperature coal tar results in connection 
with the production both of illuminating gas and of coke. The 
apparatus and methods used, although they involve the same general 
reactions, differ in the following respects : First, the coals are different. 
In general, gas coal contains from 30 to 40 per cent of volatile matter, 
while the coal used for the production of coke contains from 15 to 25 
per cent of volatile matter. Second, the sizes of the coking chambers 
are different. The capacity of the gas retort is measured by the 
hundredweight of coal per charge; the capacity of the coke oven is 
measured by the ton. The time of coking in gas-house practice may 
vary from 6 to 12 hours; in coke ovens it may vary from 24 to 60 
hours. 
There are three different types of gas benches used in this country, 
each type in general having several different subdivisions. The 
three types receive their names from the position of the retort itself 
They are the horizontal retort, the inclined retort, and the vertical 
retort. 
Horizontal-retort benches. — Horizontal retorts have a cross-section 
similar to the letter D laid on the flat side, and are about 18 inches 
wide and 15 inches high. They may be 6 to 18 feet long. The retort 
may be heated by direct coal fire, as in the older systems; or by 
producer gas, as in the more modern types. The coal may be charged 
by hand, sometimes by shovel, sometimes by a specially constructed 
trough or scoop; or by machinery, a retort known as the " through 
retort" having come largely into use. Figures 2 and 3 show views 
of a bench of machine-charged retorts. The hand-charged retort is 
usually filled about 6 inches in depth with coal, but it is possible to 
fill the machine-charged retort 12 inches deep. It has been shown 
by Lewes (2) that in the conditions first described the gas formed at 
the bottom of the retort rises through the charge and escapes from 
the retort chiefly through the space above the coal, and that it must 
of necessity come in very close contact with the heated walls of the 
retort. This almost assures a complete tertiary reaction. On the 
other hand, if the retort is filled, as it is possible for it to be with 
machine charging, the gas may escape largely through the center of 
the charge, which is cooler, and there may not be so complete a 
tertiary reaction. The tar would then not be composed entirely of 
aromatic hydrocarbons. Another factor that may enter into the 
