290 CONSIDERATION OF CARBON COMPOUNDS. 



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 turpentine, C 10 H 16 ; 

 acetylene, C 2 H 2 , is polymeric with benzene, C 6 H 6 ; acetic acid, 

 C 2 H 4 O 2 , is polymeric with grape-sugar, C 6 H 12 O 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 mole- 

 cule. Ammonium cyanate, NH 4 CNO, is easily converted into urea, 

 CO(NH 2 ) 2 . 



b. A molecule may split up into two or more molecules. For 



instance : 



C 6 H 12 6 : : 2C 2 H 6 + 2C0 2 . 

 Grape-sugar. Alcohol. Carbon dioxide. 



c. Two molecules, either of the same kind, or of different sub- 

 stances, may unite together directly : 



-f 2Br = C 2 H 4 Br 2 . 

 Ethylene. Bromine. Ethylene bromide. 



d. Atoms may be removed from a compound without replacing 

 them by other atoms : 



C 2 H 6 O + O = C 2 H 4 O + H 2 O. 

 Alcohol. Oxygen. Aldehyde. Water. 



e. Atoms may be removed and replaced by others at the same 

 time (substitution) : 



C 2 H 4 O 2 -f 2C1 : C 2 H 3 C10 2 + HC1. 



Acetic acid. Chlorine. Monochloracetic Hydrochloric 

 acid. acid. 



Action of heat upon organic substances. As a general rule, 

 organic bodies are distinguished by the facility with which they 

 decompose under the influence of heat or chemical agents ; the more 

 complex the body is, the more easily does it undergo decomposition 

 or transformation. 



Heat acts differently upon organic substances, some of which may 

 be volatilized without decomposition, whilst others are decomposed 

 by heat with generation of volatile products. This process of heating 

 non-volatile organic substances in such a manner that the oxygen of 

 the atmospheric air has no excess, and to such an extent that decom- 

 position takes place, is called dry or destructive distillation. 



