HYDROCARBONS 51 



coal seams, but was originally called marsh gas, because it was found 

 to escape from the water of ponds, where it is now known to be formed 

 by the decomposition of cellulose. By a similar process of bacterial 

 action it may be produced in the intestines of animals. The middle 

 members, containing 5-16 atoms of carbon, are liquids, and are con- 

 tained in petroleum, which consists of a mixture of saturated hydro- 

 carbons. The higher members are solids. 



Two theories have been advanced to account for their formation. 

 According to the first, they are the products of the dry distillation of 

 animal remains ; according to the second, they are formed by the action 

 of water upon the metallic carbides, of which the interior of the earth is 

 supposed to consist. If the former supposition be the correct one, as 

 the most recent work tends to show, they become of still greater interest 

 in biological chemistry. 



Several fractions are separated by the fractional distillation of the 

 natural mineral oil. The following are the principal fractions from 

 American petroleum : 



1. Cymogene, B.P. o\ gases which are liquefied by pressure and used for producing 



2. Rhigolene, B.P. i8J cold by evaporation. 



3. Petroleum ether or naphtha, B.P. 5o-6o, contains chiefly pentane and hexane. 



4. Benzoline or Benzine, B.P. 7o-go, contains chiefly hexane and heptane. 



*"<* heptane and octane. 



7. Paraffin oil or Kerosene, B.P. i5o-3oo, contains chiefly octane to hexadecane. 



8. Vaseline, B.P. above 300, contains chiefly heptadecane to heneicosane (C ZI H U ). 



The fractions may be purified by shaking with concentrated sulphuric 

 acid and caustic soda to remove unsaturated hydrocarbons. 



The portions of higher boiling-point are decomposed by overheat- 

 ing or by distilling under pressure (cracking process) and yield fractions 

 of lower boiling-point. 



The other natural mineral oils, found in Russia, Roumania, etc., are 

 also distilled fractionally. They contain generally less of the lower boil- 

 ing fractions and a greater quantity of naphthene hydrocarbons (p. 237). 



Liquid hydrocarbons are also prepared by the distillation of bitu- 

 minous shale. 



The higher members, which are solid, remain as distillation residues, 

 and are also found in nature, e.g. ozokerit. 



The distillation residues are converted into oil and paraffin wax 

 by freezing and pressing ; the liquid portion forms lubricating oil and 

 the solid portion paraffin wax. The residues and fractions may be 

 purified by treatment with sulphuric acid and caustic soda. 



