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 



