CONSTITUTION OF ORGANIC COMPOUNDS. 249 



I. II. III. IV. V. 



Hydrochloric Potassium Arsenious Ethane. Phosphoric 



acid. hydrate. chloride. oxychloride. 



m CEL TJ O f i, 



^ I > H J/ U 



H CL, K H, As Cl, C , P. . 



\ri I \w 1 Xcl 



N L1 TT Al 



The graphic representation of the constitution of compounds 

 according to types has greatly aided in disclosing their struct- 

 ure, and is frequently used to give a picture, as it were, of the 

 theoretical views held regarding the atomic arrangement. 



Substitution is a term used for those reactions or chemical 

 changes which depend on the replacement of an atom or a 

 group of atoms by other atoms or groups of atoms. Substi- 

 tution takes place in organic or inorganic substances, and its 

 nature may be illustrated by the following instances : 



K + H 2 = KHO + H. 



Potassium. Water. Potassium Hydrogen. 



hydrate. 



C 2 H 4 2 + 2C1 = C 2 H 3 C10 2 + HC1. 



Acetic acid. Chlorine. Monochlor- Hydrochloric 



acetic acid. acid 



C 6 H 6 + H]S T 3 = C 6 H 5 N0 2 + H 2 0. 



Benzine. Nitric acid. Nitro-benzine. Water. 



Derivatives. This term is applied to bodies derived from 

 others by some kind of decomposition, generally by substitu- 

 tion. Thus, nitro-benzine is a derivative of benzine; chloroform, 

 CHC1 3 , is a derivative of methane, CH 4 , obtained from the 

 latter by replacement of three atoms of hydrogen by the same 

 number of atoms of chlorine. 



Isomerism. Two or more substances may have the same 

 elements in the same proportions by weight (or the same cen- 

 tesimal composition), and yet be different bodies, showing dif- 

 ferent properties. Such substances are called isomeric bodies. 

 Two kinds of isomerism are distinguished, viz., metamerism 

 and polymerism. 



