THE PRINCIPLES OF OEGANIC CHEMISTRY 51 



position under defined conditions, but they cannot be looked 

 upon as chemical individuals in the same sense as compounds 

 which fulfil one of the above-mentioned requirements. 



The determination of the constitutional formula of the 

 countless substances met with in the study of organic chemistry 

 shows that they can be classified under three heads. 



1. Aliphatic compounds, viz., all open chain compounds both 

 saturated and unsaturated, viz., the paraffin, olefine, acetylene, 

 etc., hydrocarbons already referred to, and their derivatives. 



2. Isocyclic compounds. All compounds containing closed 

 chains formed by the union of carbon atoms only, viz., deri- 

 vatives of polymethylene hydrocarbons, consisting of rings 

 formed by three or more CHg groups ; thus : 



CH 



trimethylene 



or substances derived from benzene 



CH 



CH 



and from hydrocarbons containing more than one ring such 

 as naphthalene, anthracene, etc. 



3. Heterocyclic compounds. All compounds containing 

 closed chains, having other atoms in addition to carbon atoms, 

 viz. : 



PTT 

 HC CH _X 



L pyridine 



thiophene 



HC CH 



V 



N 



2 



