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 henches. — 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 

 illl 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 



