CHAPTER II. COMPOSITION OF TARS AND METHODS OF MANUFACTURE. 
The character of the tars from which creosotes are made determines 
in the main the character of the creosote. The methods used in pro- 
ducing the oil may have a slight effect upon the character of the 
creosote if the tar contains only small amounts of the " paraffin" 
bodies, but those methods would not change a low-temperature tar 
to a high-temperature tar. These two classes of tar are determined 
by the methods used in producing the tar itself. A short description 
of the method of manufacturing tar and of the general character of 
the tar as to both its chemical and physical properties is, therefore, 
very essential as a basis for the discussion of creosotes. Tars of all 
descriptions are usually by-products of the manufacture of other 
materials and, consequently, are subject to wide variation, for the 
reason that, as a rule, no attempt is made to control their composi- 
tion other than for the purpose of facilitating their removal. 
COAL TAR. 
Coal tar is obtained by the destructive distillation of bituminous 
or semibituminous coal. The main products of this reaction are coke 
and gas; the chief by-products, ammonia and tar liquor. Coal is 
generally distilled either for the production of illuminating gas or for 
the production of metallurgical coke. Whichever product is desired, 
the other is usually considered of secondary importance, and all the 
conditions are regulated to give a maximum yield of the highest 
quality of the main product. Whether gas or coke is to be the main 
product, there are differences in the character of the coal used, 
differences in the amount of coal used in each charge, differences in 
the time taken to carbonize, and differences in the temperatures 
used. A few coke plants are so situated that they have a demand 
for gas as well as coke, and in them both materials are principal 
products. Under such conditions slightly different methods of 
operation may be followed. However, the fundamental conditions 
governing the distillation of coal will always apply. 
COMPOSITION AND DESTRUCTIVE DISTILLATION OF COAL. 
Considerable work has been done upon the composition of various 
coals, and perhaps the most simple explanation of the different 
