8 
BULLETIN i036, U. S. DEPARTMENT OE AGRICULTURE. 
products obtained from coal is given by Lewes (2), 1 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 3A 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. 
lwater. 
[Thin tar. 
Humous bodies 
VFree carbon. 
[Methane 
[Carbon monoxide 
Free carbon. 
Pitch. 
/Free carbon. 
\Pitch. 
Resin bodies 
Hydrocarbons 
^Carbon dioxide 
(Ethylene and other unsaturated hydrocarbons 
Methane, ethane, and other paraffins 
(Thin tar 
Heavy tar 
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. 
i All italic figures inclosed within parantheses refer to th3 bibliography, in which numbers are assigned 
to the different articles of literature. 
