COAL-TAR AND WATER-GAS TAR CREOSOTES. 13 
composition of tar is the relation of the equipment at the gas plant 
to the demand for gas. The most efficient manner of operating any 
plant is not always the most expedient. .During the war, when it 
was hard or impossible to obtain machinery, and at the same time 
the demand for the manufactured products was great, by shortening 
the time of coking and not attempting to remove all the gas, it was 
then found possible to increase the output of a given equipment to a 
considerable extent. Under such conditions the tar would be pro- 
duced at a lower average temperature and hence the paraffin content 
would be higher. 
Inclined-retort benches. — In the inclined-retort system the retorts 
are set at an angle which is as near as possible to the angle of recline 
of coal. This is in the neighborhood of 30 degrees. Coal is charged 
by gravity at the upper end of the retort, and under ideal conditions 
fills the retort to an even depth for its full length. This condition is 
not always realized in practice, and not infrequently a considerable 
amount of unburned coal is found near the lower door, while the upper 
part of the retort is nearly bare. This results in the tar containing 
high amounts of paraffin. At the present time the use of this system 
is not extending in this country. 
Vertical-retort benches. — The fact that much space in textbooks is 
being devoted to the vertical retort would seem to indicate that in 
time this type of retort may replace the horizontal for gas-making 
purposes. The retorts are oval in cross-section and slightly larger 
at the bottom than at the top. The coal is charged and the coke 
extracted by gravity. For this reason the coal completely fills the 
cross-section of the retort. The escaping gases are, therefore, 
forced to travel chiefly through the cool core of uncarbonized coal. 
The resulting tars contain considerable amounts of paraffin bodies. 
In general, the specific gravity of tars from vertical retorts is less than 
that of tars from horizontal rerorts. 
By-product coke ovens. — There are several types of by-product 
coke ovens in this country, but by far the greater proportion of them 
belongs to one or another of three systems. These systems differ 
from each other in several ways, as in methods of heating, in methods 
of regeneration of heat, and in methods of recovery of by-products, 
but the general principle underlying the manufacture of coke is the 
same as that used in retort practice. 
The three principal systems are the Otto system, with its various 
modifications, the Semet-Solvay system, and the Koppers system. 
Figures 4, 5, 6, 7, and 8 show views of these three types of oven. 
By-product coke ovens are, in general, long, rectangular ovens which 
may vary in height from 3 to 9 feet and may be as long as 35 feet. 
The width of the oven, however, varies within narrow limits, being 
17 to 19 inches. Inasmuch as coke ovens are generally erected in 
