250 CONSIDERATION OF CARBON COMPOUNDS. 



Metamerism. Substances are metameric when their molecules 

 contain 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 only explanation which can be given regarding this 

 difference of properties is, that the atoms are arranged differ- 

 ently within the molecule. In some cases this arrangement is 

 as yet unknown, in other cases structural or graphic formulas 

 showing this atomic arrangement may be given. 



For instance : Acetic acid and methyl formate have both 

 the composition C 2 H 4 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\ CHO\ Q 



H/ U " CH,/ 



As another instance, may be mentioned the compound 

 CTN~ 2 H 4 O, which represents either ammonium cyanate or urea : 



Ammonium cyanate. Urea. 



NH 4 \ NH 2\CO 



ON/ ' NH 2 X LU ' 



Polymerism. Substances are said to be polymeric when they 

 have the same centesimal composition, but a different molecular 

 weight, or, in other words, when one substance contains some 

 multiple of the number of each of the atoms contained in the 

 molecule of the other. 



For instance, some volatile oils have the composition, C 20 H 32 , 

 which is double the number of atoms contained in oil of tur- 

 pentine, C 10 H 16 ; acetylene, C 2 H 2 , is polymeric with benzine, 

 C 6 H 6 ; acetic acid, C 2 H 4 O 2 , is polymeric with grape-sugar, 

 C 6 H 12 6 , etc. 



' Various modes of decomposition. The principal changes which 

 a molecule may suffer are as follows : 



a. The atoms may arrange themselves differently within the 

 molecule. Ammonium cyanate, NH 4 ClSrO, is easily converted 

 into urea, CO2(N"H 2 ). 



