BOILING POINT AND CONSTITUTION 53 



then follows 



N 2 O 4 26, 



and finally N.,O 5 , 



solid at ordinary temperatures and decomposed above 50. 



(c] Equal Differences in Boiling Point for Equal 

 Differences in Composition. 



It is often found that equal differences of composition 

 produce equal differences in boiling point, as is generally 

 the case among homologous bodies such as those differing 

 by CH 2 . Taking the esters as examples : 



The effect of CH 2 is to cause a difference of from 20-8 to 

 25 or, as usually, not far from 20. 



(d) Compounds of more or less Volatile Elements. 



On replacing one element in a compound by another the 

 relative volatility of the two often causes a change in 

 boiling point, e. g. 



H, - 238 F 2 - 185 C1 2 - 33 Br 2 59 I 2 184 



C 6 H 6 80-4 C 6 H 5 F 85 C 6 H 5 C1 132 C 6 H 5 Bri 55 C 6 H 5 I 190 



In connexion with this is the fact that though increase 

 in the size of the molecules causes in general a rise of 

 boiling point, addition of the very volatile hydrogen causes 

 hardly any changes, or sometimes a fall : 



Benzene (C 6 H 6 ) 80-4 Hexamethylene (C<-H 12 ) 70. 



(e) Molecular Weight and Boiling Point. 



It follows from the additive character that an abnormal 

 increase of molecular weight is of the greatest influence on 



