344 



perature. The members of the series from pentane to penta- 

 decane (Ci 5 H 3 2) are liquid under ordinary conditions. From 

 hexadecane onwards they are solids, with higher and higher melt- 

 ing-points. 



In these compounds the carbon is quadrivalent, and each sub- 

 stance is related to the preceding one by containing the additional 

 units CH 2 . The formulae of the first three members may be writ- 

 ten graphically to illustrate these two facts : 



H H H H H H 



I II III 



H-C-H H-C-C-H H-C-C-C-H 



I II III 



H H H H H H 



Petroleum. Petroleum is a thick, greenish-brown oil. When 

 borings are made into the oil-bearing strata, the oil either gushes 

 up, or is pumped to the surface. In the United States many thous- 

 ands of miles of pipe-lines are used to transport the oil, with the 

 aid of force pumps, to the refineries, and in 1920 nearly 450 mil- 

 lion barrels (42 gal. each) were produced. The world's produc- 

 tion in 1920 was 700 million barrels. 



After the United States, Mexico and Russia are the chief 

 producers of petroleum. 



Oil Refining. The natural oil is a complex mixture, and is 

 partially separated by distillation (p. 67) into products which 

 are still mixtures, but are suited to special purposes. The com- 

 ponents of lower boiling-point come off first and the temperature 

 rises steadily as these components are eliminated and those of 

 higher and higher boiling-point enter the vapor. As certain 

 temperatures are reached (or as the sp. gr. of the distillate attains 

 certain values) the condensed liquid is diverted into different 

 vessels, so as to collect together the " fractions " of the same kind. 

 This is called fractional distillation. 



