298 CONSIDERATION OF CARBON COMPOUNDS. 



40. HYDEOCAEBONS. 



Occurrence in nature. Hydrocarbons are seldom derived from 

 animal sources, being more frequently products of vegetable life; 

 thus, the various essential oils (oil of turpentine and others) of the 

 composition C 10 H 16 or C 20 H 32 are frequently found in plants, where 

 they are formed from carbon dioxide and water : 



10C0 2 + 8H 2 : C 10 H 16 + 280. 



This equation, whilst showing the possibility of the formation of 

 an essential oil in the plant, must not be taken to mean that 10 

 molecules of carbon dioxide and 8 molecules of water are simultane- 

 ously decomposed, with the production of a hydrocarbon; on the 

 contrary, we know that many intermediate substances are formed, 

 and the formula simply gives the final result, not the intermediate 

 stages of the process. 



Other hydrocarbons are found in nature as products of the decom- 

 position of organic matter. Thus methane, CH 4 , is generally formed 

 during the decay of organic matter in the presence of moisture ; the 

 higher members of the methane series are found in crude coal-oil. 



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 electric sparks pass between electrodes of carbon in an atmos- 

 phere of hydrogen. 



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 decompo- 

 sition. 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 sub- 

 stances. 



The hydrocarbons found in nature are generally separated from 

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



terms isomerism, metamerism, and polymerism. 385. How does heat act upon 

 organic compounds? 386. What is destructive distillation? 387. State the 

 difference between combustion, decay, fermentation, and putrefaction ; what is 

 the nature of these processes, and under what conditions do they take place ? 



388. How do chlorine, nitric acid, and alkalies act upon organic substances? 



389. What is the action of hydrogen, and of dehydrating agents, upon organic 

 substances ? 390. Mention the chief groups of organic compounds. 



