CONSTITUTION OF ORGANIC COMPOUNDS. 289 



other atoms or groups of atoms. Substitution takes place in organic 

 or inorganic substances, and its nature may be illustrated by the fol- 

 lowing instances : 



K + H 2 O = KOH + H. 

 Potassium. Water. Potassium Hydrogen, 

 hydroxide. 



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



Acetic acid. Chlorine. Monochloracetic Hydrochloric 

 acid. " acid. 



C 6 H 6 + HN0 3 : C 6 H 5 N0 2 + H 2 O. 

 Benzene. Nitric acid. Nitro-benzene. Water. 



Derivatives. This term is applied to bodies derived from others 

 by some kind of decomposition, generally by substitution. Thus, 

 nitro-benzene is a derivative of benzene; 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 proportion' by weight (or the same centesimal composi- 

 tion), and yet be different bodies, showing different properties. Such 

 substances are called isomeric bodies. Two kinds of isomerism are 

 distinguished, viz., metamerism and polymerism. 



Metamerism. Substances are metameric when their molecules con- 

 tain equal numbers of atoms of the same elements. Thus, the oils 

 of juniper, turpentine, lemon, etc., all have the molecular formula 

 C 10 H 16 , and yet they have different physical properties, and may be 

 distinguished by their odor, by their action on polarized light, etc. 



The explanation given regarding this difference of properties is, 

 that the atoms are arranged differently within the molecule. In 

 some cases this arrangement is as yet unknown, in other cases struc- 

 tural or graphic formulas showing this atomic arrangement may be 

 given. 



For instance : Acetic acid and methyl formate both have the com- 

 position C 2 H 4 O 2 , but the arrangement of the atoms (or the structure) 

 is very different, as shown by the formulas : 



Acetic acid. Methyl formate. 



C 2 H 3 0\ Q CHO\ 



H/ U CH 3 /' 



As another instance may be mentioned the compound 

 which represents either ammonium cyanate or urea : 



Ammonium cyanate. Urea. 



NH 2 \nn 

 NHJ/ 00 ' 



19 



