SYNTHETICAL DRUGS 573 



lating in the molecules of these substances. In certain organic 

 compounds the carbon atoms are arranged in chains which 

 are either straight or branching. If we consider one of these 

 chains having six carbon atoms, and if we suppose further 

 that the chain is coiled so that the ends meet like a snake with 

 its tail in its mouth, we then get a representation of the carbon 

 backbone of the benzene molecule. To each of these carbon 

 vertebrae one of the hydrogen atoms is attached ; and now to 

 give the finishing touch to our benzene ring we must consider 

 what is the usual habit of combination of carbon in the many 

 substances in which it is present. Of the thousands of carbon 

 compounds known there are scarcely any which furnish excep- 

 tions to the general rule that this element is quadrivalent, or in 

 other words that its atom has four bonds or valencies with 

 which to attach itself to other atoms. 



The following graphical formula I. for benzene indicates 

 one of several ways in which this condition is satisfied, this 

 particular mode of representation being due to Kekule : 



The double linkings represent points of great reactivity 

 in the molecule, and their presence exerts a very profound 

 influence on the chemical properties of benzene and its deriva- 

 tives. It is, moreover, largely owing to these double linkings 

 that benzene derivatives are so frequently of therapeutic im- 

 portance. The physiological action of benzene itself is, however, 

 only slight, the compound having merely the depressing hypnotic 

 effect characteristic of the hydrocarbons in general. 



Antipyretics 



Aniline (formula III.) is derived from benzene by first treating 

 the hydrocarbon with nitric acid whereby nitrobenzene (formula 

 II.) is produced and then replacing the oxygen atoms in the 

 latter compound by two hydrogen atoms. This process is 



