COAL-TAR AND WATER-GAS TAR CREOSOTES. 
41 
is less difference in the results from using different types of retort 
or oven than is often shown in the use of different ovens or retorts of 
the same type. In other words, if the coal and the temperature of 
coking are the same, the type of retort or oven seems to have little 
or no effect. This seems to indicate that the producing of highly 
aromatic coal-tar creosotes is dependent upon the temperature of 
coking. If this is true, then the method of applying the heat to the 
retort may have some effect upon the quality of the coal-tar creosote. 
According to some researches at the University of Wisconsin (14) 
on producer gas, tar is formed from coals at comparatively low 
temperatures, ranging from 200° to 600° C. This is far below the 
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Fig. 21.— Summaries of the physical and chemical measurements of fractions of all authentic coal-tar 
creosotes. 
A. Index of refraction values. 
B. Specific gravity values. 
1. Horizontal-retort tar creosotes. 
2. Coke-oven tar creosotes. 
3. Inclined and vertical retort tar creosotes. 
C Sulphonation residues. 
temperature of coking in both the by-product and gas-house retorts. 
Table 8 shows that the temperature of coking in the by-product oven 
rarely goes below 900° C. and that the average is about 1,000° C. If 
tars are produced at temperatures as low as 200° C. and are com- 
pletely given off from the coal at 600° C, and if the retort is heated to 
a point as high as 1,000° C, the tar vapor, as it rises from the coal, 
must come into contact to a certain extent with a surface heated far 
above its temperature of formation, when it may, and probably does, 
undergo a chemical change. It is well known that paraffin-like oils 
may be obtained if coal is coked at low temperatures. It is also 
significant to note that in this work the tars produced at the highest 
temperatures yielded either no coal-tar creosotes at all or else one of 
