CONSTITUTION OF ORGANIC COMPOUNDS. 451 



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. Three kinds of isomerism are 

 distinguished, viz., metamerism, polyinerism, and stereo-isomerism. 



Metamerism. Substances are metameric when their molecules con- 

 tain equal numbers of atoms of the same elements. Thus, cane- 

 sugar and milk-sugar have both the composition C 12 H 22 O n , and yet 

 they have different physical properties, and may be distinguished by 

 their solubility and by a number of characteristic tests. 



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 S 0\ CHO\ 



a 0< 



CH 3 X 



As another instance may be mentioned the compound CN 2 H 4 O, 

 which represents either ammonium cyanate or urea : 



Ammonium cyanate. Urea. 



NH 4 \ NH A CO 



CN/ U NH 2 / OU ' 



Polymerism. Substances are said to be polymeric when they have 

 the same centesimal composition, but a different molecular weight, or. 



