116 POWER AND THE PLOW 



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If crude oil is left standing at ordinary temperatures a part 

 of it will be given off as vapor. On a warm day more vapor 

 will be given off before evaporation stops than on a cold one. 

 Likewise, if the oil is placed on a stove more gas is given off and 

 the remaining liquid grows denser. At each rise in temperature 

 more of the oil is vaporized, until nothing is left but a solid 

 residuum. This behavior shows that petroleum is not a uni- 

 form substance, but a mixture composed of large numbers of 

 different compounds. ' In order to separate the oil into liquids 

 which are uniform in quality a process similar to the one just 

 described is used, this being known as fractional distillation. 

 It consists of applying successively higher temperatures to 

 the crude oil to vaporize the various compounds, which are 

 again condensed, provided, of course, they are liquid at ordi- 

 nary temperatures. Each time evaporation stops, the tem- 

 perature of the liquid is raised and another portion is distilled 

 over, until finally the volatile matter is all driven off. 



From the foregoing it will be seen that petroleum contains: 



(1) some compounds which are gases at ordinary temperatures; 



(2) some which are normally liquid when confined, though 

 evaporating quickly when exposed; (3) some which are liquid 

 and require considerable heat to vaporize; (4) some which are 

 normally liquid, vaporizing only with difficulty and at very 

 high temperatures; and (5) some which for practical purposes 

 are not volatile at all. The first group, roughly speaking, 

 contains the natural gases. The second comprises the grades 

 variously known as gasoline, benzine, naphtha, petroleum spirit, 

 petrol, etc., and a few rare products not of commercial importance 

 such as rhigolene and cymogene. The third group is made up 

 of illuminating oils, kerosene, paraffin oil, etc. "Middlings," 

 or "distillate," comes under this head, though as a matter of 

 fact all the foregoing products are distillates of crude oil. 

 The fourth group contains the heavy fuel, gas, and lubricating 

 oils, while the fifth comprises the solid lubricants, paraffin 

 wax, petroleum coke, pitch, asphalt, etc. 



