on the Physical Properties of Compounds. 115 



replacing one or more of the atoms of hydrogen by the atoms 

 or groups of atoms of other elements. 

 The ring 



C 



/l % 

 -Ce 2C- 



i 

 -C5 30- 



\ 



is called the benzene ring, or the benzene nucleus : and the 

 carbon atoms in this ring we will number, for sake of distinc- 

 tion, as in the figure. 



Xow suppose we replace one atom of hydrogen by an atom 

 of another element, say bromine : then we can only get one 

 derivative, for an identical compound is obtained no matter 

 which of the six atoms of hvdrogen is replaced by the atom of 

 bromine, thus : — 



Br 



5 3 

 4 



&c. 



B 



But when we replace two atoms of hydrogen by two like 

 atoms of another element, or by two like groups of elements, 

 then no less than three different isomeric bodies may be ob- 

 tained: viz. — 



Br 

 (6 2 



Br 





Br 

 16 2 







Br 



M 



lo 3 

 \ 4 / 







Is 3 y 



\4/ 



Br 





L si 



- or 1.2-cb 

 benzena 



broni- 



Meta- 



or 1.3-di 



brom- 



Paia 



Br 

 • or 1.4-dibrom- 



benzene. 



It has been observed for some time past, that para-compounds 

 always melt higher than ortho- and m eta-compounds: whereas, 

 as regards the ortho- and meta-compounds, the former some- 

 times melt higher, and sometimes lower, than the latter. 



Xow the arrangement of the atoms in the para-compounds 

 is symmetrical, whilst in the ortho- and meta-compounds the 



