HYDROCARBONS. 259 



QUESTIONS. 



371. Explain the term residue or radical. 



372. What is understood by the expression chain, when used in 

 chemistry? 



373. What are the characteristics of a homologous series? 



374. Give an explanation of the terms isomerism, metamerism, and 

 polymerism. 



375. How does heat act upon organic compounds ? 



376. What is destructive distillation? 



377. State the difference between combustion, decay, fermentation, 

 and putrefaction; what is the nature of these processes, and under 

 what conditions do they take place? 



378. How do chlorine, nitric acid, and alkalies act upon organic sub- 

 stances? 



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

 organic substances? 



380. Mention the chief groups of organic compounds. 



39. HYDROCARBONS. 



Occurrence in nature. Hydrocarbons are seldom derived from 

 animal sources, being generally 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 : 



10CO 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 simultaneously decomposed, with the production of an 

 essential oil ; 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 

 decomposition of organic matter. Thus methane, CH 4 , is gen- 

 erally formed during the decay of organic matter in the pres- 

 ence of moisture; the higher members of the methane series are 

 found in crude coal-oil. 



