COAL-TAR A'jSTD WATER-GAS TAR CREOSOTES. 9 



Table 2. — Hydrocarbons foundin the primary-reaction products of the distillation of coal. 



Saturated hydrocarbons. 



Methane CH4 



Ethane C^Hr 



Propane CsHs 



Butane C4H10 — 



Pentane C6H12 



Hexane CsHii — 



Heptane C7H1S 



Octane CsHis 



Monane CjIIso. . . 

 Decane C10H22... 

 Possibly paraffins 



Unsaturated 

 hydrocarbons. 



Naphthenes. 



Ethylene 



Propylene 



Butylenc 



Amyiene 



Hexylene 



Heptylene 



C4H8. 



CbHio- 

 C6H12. 

 CvHu. 



Hexahydrobenzene CsHis. 



All these hydrocarbons, both gaseous and liquid, are similar to 

 what we obtain from crude petroleum. In addition to the above, 

 the oxygen in the coal is probably in a large measure converted into 

 oxides of carbon and phenols, as we find these materials in great 

 abundance in low-temperature tars. All of the products of the 

 primary reaction, with the exception of the gaseous carbon monoxide, 

 carbon dioxide, methane, and ethane, will undergo decomposition at 

 somewhat higher temperatures into more simple hydrocarbons 

 with a production of the permanent gases, methane, ethane, ethylene, 

 acetylene, and hydrogen. This is the secondary reaction. The 

 products of the secondary reaction probably contain compounds 

 having three carbon atoms or less, with the exception of possibly 

 small amounts of benzene and xylene, which result from the decom- 

 position of the naphthenes. We should expect that this phase of 

 the reaction would produce chiefly hydrogen; the saturated hydro- 

 carbons — methane, ethane, and possibly propane; and the unsatu- 

 rated hydrocarbons — ethylene, propylene, acetylene, and amyiene. 

 All of these, as well as different members of the acetylene series, have 

 been identified as products of the distillation of coal. If the gas 

 maker could so arrange his distillation that only the first and second 

 reactions would take place, it would probably be a very satisfactory 

 arrangement for him. In order, however, to heat the coal hot 

 enough all the way through to induce a secondary reaction as nearly 

 complete as possible, it is necessary to superheat the retorts or ovens, 

 and, as a result, a third reaction may and does take place. 



The tertiary reaction consists in building up more complex but 

 more stable compounds from the more simple ones. This reaction 

 is often accompanied by the elimination of hydrogen. The hydro- 

 carbons formed are those classed by the chemist as aromatic hydro- 

 carbons, and they differ from the paraffin hydrocarbons in containing 

 less hydrogen per carbon atom, in the arrangement of the carbon 

 atoms themselves, in their chemical reactions, and in their resistance 

 to heat. The aromatic hydrocarbons may be subdivided into a 



