150 ARTIFICIAL LIGHTING 



condensers and tanks to receive various distillates are used. 

 When heat is applied to the crude liquid in the retort, the 

 various constituents are affected differently; the substances 

 in the crude petroleum which have a low boiling point vaporize 

 first and distill over into the condensers. As the temperature 

 of the liquid in the retort rises, substances with higher boiling 

 points distill over and condense ; as the temperature rises 

 still higher, various other compounds are driven off, and at a 

 very high temperature all that is left in the retort is a dark, 

 thick mass called coke. 



The liquids which distill over at different temperatures 

 are unlike in character, and serve widely different purposes. 

 Among the products obtained in this way by the fractional 

 distillation of crude petroleum are benzine, gasoline, kerosene 

 or coal oil, lubricating oils, petrolatum, and paraffin. Many 

 of the products thus obtained require further treatment before 

 they are of real commercial value ; kerosene, for example, is 

 freed as far as possible of its objectionable odor. 



144. Illuminating Gas. Much of the gas which illuminates 

 our houses is made from the distillation of bituminous coal. 

 Soft coal is placed in clay-lined retorts which are connected 

 by pipes to a series of tanks. When the coal is heated to 

 1200 C. or more, certain substances in it volatilize and pass 

 through an exit tube into a trough which contains water, and 

 is called the hydraulic main. The water in the hydraulic 

 main condenses some of the tarry matter which distilled over 

 from the coal, and purifies the uncondensed and insoluble 

 gases which bubble through it. From the hydraulic main, 

 the uncondensed and insoluble gaseous products pass onward 

 through a series of coils where they are cooled, and where, as 

 a result of cooling, further condensation occurs. The gas, 

 which at the lower temperature still remains uncondensed, 

 passes through a series of vessels which possess devices for 



