CLASSIFICATION OF ORGANIC COMPOUNDS. 297 



by replacement of the hydrogen of carboxyl by alcohol radicals. 

 For instance : 



C 2 H 5 \ CH 3 CO\ C 2 H 5 \ , H\ 



H/ C H/ C = CH 3 CO/ C ~ H/ C 



Ethyl alcohol. Acetic acid. Acetic ether. Water. 



The various fats belong to this group of compound ethers. 



7. Carbohydrates. (Sugars, starch, cellulose, etc.) These com- 

 pounds contain 6 atoms of carbon (or a multiple of 6) in the molecule, 

 and hydrogen and oxygen in the proportion of 2 atoms of hydrogen 

 to 1 atom of oxygen, or in the proportion to form water. Most 

 carbohydrates are capable of fermentation, or of being easily con- 

 verted into fermentable bodies. Instances : C 6 H 12 O 6 , C 6 H 10 O 5 , etc. 



Glucosides are substances the molecules of which may be split up 

 in such a manner that several new bodies are formed, one of which 

 is sugar. 



8. Amines and amides. Substances formed by replacement of 

 hydrogen in ammonia by alcohol or acid radicals. For instance : 

 ethyl amine, NH 2 .C 2 H 5 , urea, N 2 H 4 .CO, etc. The alkaloids belong 

 to this group. 



9. Cyanogen and its compounds. Substances containing the radical 

 cyanogen, CN. For instance : potassium cyanide, KCN. 



10. Proteids or albuminous substances. These, besides carbon, 

 hydrogen, and oxygen, always contain nitrogen and sulphur, some- 

 times also other elements. Instances : albumin, casein, fibrin, etc. 



In connection with each of these groups have to be considered the 

 derivatives obtained from them directly or indirectly. 



As all those organic compounds the constitution of which has 

 been explained may be looked upon as derivatives of either methane, 

 CH 4 , or benzene, C 6 H 6 , a separation of organic compounds is made 

 into two large classes, each one embodying all the derivatives of one 

 of the two hydrocarbons named. The derivatives of methane are 

 termed fatty compounds, those of benzene aromatic compounds. Fatty 

 compounds have representatives in each one of the above ten groups : 

 aromatic compounds are missing in a few. As far as practicable, the 

 two classes will be considered separately, because the properties of 

 fatty and aromatic compounds differ so widely, in some respects, that 

 this method of studying the nature of carbon compounds is to be 

 preferred. 



QUESTIONS. 381. Explain the term residue or radical. 382. What is under- 

 stood by the expression chain, when used in chemistry ? 383. What are the 

 characteristics of an homologous series? 384. Give an explanation of the 



