32 BACTERIOLOGICAL CHEMISTRY 



Amide — cf 13,200 



\NH2 



The nitro group, — NO2, is also polar, and is of approxi- 

 mately the same strength as the hydroxyl group as it 

 occurs in alcohols. The numerical values given, which 

 indicate the comparative strength of the groups, represent 

 the energy (in calories per gram molecule) required 

 to separate the molecules from one another and are 

 calculated from the heat of evaporation of compounds 

 containing them. The more strongly polar the group 

 the greater is the attraction between molecules con- 

 taining the group, and in consequence the greater the 

 energy required to separate the molecules from one 

 another on conversion from the liquid to the vapour 

 state. 



