306 CONSIDERATION OF CARBON COMPOUNDS. 



may be ignited, a dark, almost black liquid (tar), which condenses in the tube 

 B, and that a residue is left which is chiefly carbon. The tarry liquid shows an 

 acid reaction, due to acetic and other acids present. 



Coal-tar, obtained as a by-product in the manufacture of illumi- 

 nating gas, contains, as shown by the above table, many valuable sub- 

 stances, such as benzene, aniline, carbolic acid, paraffin, etc., which 

 are separated from each other by making use of the difference in their 

 boiling-points and specific gravities, or of their solubility or insolu- 

 bility in various liquids, or, finally, of their basic, acid, or neutral 

 properties. 



defines. The hydrocarbons of the general formula C n H 2n are 

 termed olefines. To this series belong : 



Ethene or ethylene . . . . C 2 H 4 . 



Propene or propylene .... C 3 H 6 . 



Butene or butylene C 4 H 8 . 



Pentene or amylene C 5 H 10 . 



Hexene or hexylene C 6 H 12 . 



Methene, CH 2 , the lowest term of this series, is not known. The 

 hydrocarbons of this series are not only homologous, but also poly- 

 meric with one another. 



Of special interest is the first known member of the series, ethene or 

 oleftant gas, on account of its normal occurrence in illuminating gas, 

 as well as in most common flames, the luminosity of which depends 

 greatly on the quantity of this compound present in the burning gas. 



Benzene series, or aromatic hydrocarbons. The members of a 

 series of hydrocarbons having the general composition CJET 2 - 6 , and 

 all the derivatives of this group, including the alcohols, acids, etc., are 

 the substances spoken of before as aromatic compounds, and will be 

 considered later. 



Volatile or essential oils. The term essential oil is more a phar- 

 maceutical than chemical term, and is used for a large number of 

 liquids obtained from plants, and having in common the properties 

 of being volatile, soluble in ether and alcohol, almost insoluble in 

 water, and having a distinct and in most cases even highly character- 

 istic odor. They stain paper as do fats or fat oils, from which they 

 differ, however, by the disappearance after some time of the stain 

 produced, while fats leave a permanent stain. 



In their chemical composition essential oils differ widely ; some 



