260 CONSIDERATION OF CARBON COMPOUNDS. 



Formation of hydrocarbons. It is difficult to combine the two 

 elements carbon and hydrogen directly; as an instance of such 

 direct combination, may be mentioned acetylene, C 2 H 2 , which is 

 formed when carbon is strongly heated by an electric current in 

 hydrogen gas. 



Many hydrocarbons are obtained by destructive distillation of 

 organic matter, and their nature depends on the composition of 

 the material used and upon the degree of heat applied for the 

 decomposition. Hydrocarbons may also be obtained by the 

 decomposition (other than destructive distillation) of numerous 

 organic bodies, such as alcohols, acids, amines, etc., and from 

 derivatives of these substances. 



The hydrocarbons found in nature are generally separated 

 from other matter, as well as from each other, by the process 

 known as fractional distillation. As the boiling-points of the 

 various compounds differ more or less, they may be separated 

 by carefully distilling off the compounds of lower boiling-points, 

 while noting the temperature of the boiling liquid by means of 

 an inserted thermometer, and changing the receiver every time 

 an increase of the boiling-point is noticed. This separation of 

 volatile liquids, known as fractional distillation, is, however, not 

 absolutely complete, because traces of substances having a higher 

 boiling-point are simultaneously volatilized with the distilling 

 substance. 



Properties of hydrocarbons. There are no other two elements 

 which combine together in so many proportions as carbon and 

 hydrogen. Several hundred hydrocarbons are known, many of 

 which form either homologous series or are metameric or poly- 

 meric. 



Hydrocarbons occur either as gases, liquids, or solids. If the 

 molecule contains not over 4 atoms of carbon, the compound is 

 generally a gas at the ordinary temperature; if it contains from 

 4 to 10 or 12 atoms of carbon, it is a liquid; and if it contains a 

 yet higher number of carbon atoms, it is generally a solid. 



All hydrocarbons may be volatilized without decomposition, 

 all are colorless substances, and many have a peculiar and often 

 characteristic odor; they are generally insoluble in water, but 

 soluble in alcohol, ether, bisulphide of carbon, etc. 



In regard to chemical properties, it may be said that hydro- 





