8 BULLETIN 1036, U. S. DEPARTMENT OF AGRICULTURE. 



products obtained from coal is given by Lewes (2),^ who considers 

 that all coals are composed of — 



A. Carbon residuum. C. Resin bodies. 



B. Humous bodies. D. Hydrocarbons. 



The proportion in which these four components are mixed can be 

 made to account for all the various coals known. He considers, for 

 instance, Nottingham coal as made up of A B C D; coking coals in 

 general as made up of 3 A B C 3D; anthracite as being made up of 

 5A 2C. Three of these four constituents, namely, the humous bodies, 

 the resin bodies, and the hydrocarbons, when subjected to heat, are 

 partially or completely decomposed. The temperature at which this 

 decomposition begins is probably between 300° and 400° C. The 

 problem of tar formation is, however, complicated by the fact that 

 most coal distillation is carried out in a retort or oven heated to ap- 

 proximately 1,000° C. This high temperature makes the composi- 

 tion of the tar very complicated, as it affords an opportunity for at 

 least three different reactions to take place in the same coking cham- 

 ber at approximately the same time. The primary reaction is a 

 decomposition of the coal into those products which are formed by 

 low-temperature distillations; the secondary reaction is a decom- 

 position into simple products of some of the more complicated 

 products of the primary reaction, and the tertiary reaction consists 

 of a recombination of the simple products into aromatic hydrocarbons 

 which are stable at high temperatures. These reactions are going on 

 at the same time and may be complete or only partially so, depending 

 upon several conditions. The various constituents of coal, according 

 to Lewes, yield the primary-reaction products shown in Table 1. 



Table 1. — Primary-reaction products of coal. 



Gaseous products. 



Liquid 

 products. 



Solid 

 products. 



Humous bodies 



Resin bodies 



Hydrocarbons.. 



Carbon monoxide 



Carbon dioxide 



Methane 



Carbon monoxide 



Carbon dioxide 



Ethylene and other unsaturated hydrocarbons 



Methane, ethane, and other ])araffins 



Water. 

 'Thin tar. 



Water 



Thin tar. 



J-Free carbon. 



Free carbon. 

 Pitch. 



H-ytar {R-r"""" 



The primary reaction may give the hydrocarbons shown in Table 

 2, all of which have been found in the distillation products of coal 

 decomposed at low temperatures. 



1 All italic figure.s inclosed within parantheses refer to ths bibliograiihy, in whiili numbers are assigned 

 to the different articles of literature. 



