ANNIVERSARY ADDRESS. 66 



able facts, which bear on the question of their constitution. Two 

 simple examples will suffice to illustrate the matter to those who 

 are not chemists. In ethyl-alcohol, molecularly C 2 H 6 0, the oxygen 

 cannot be removed without at the same time taking away the 

 active hydrogen, and hence the latter must be linked to the oxygen 

 thus, — O — H, oxygen being divalent. Since carbon is quadri- 

 valent and hydrogen monovalent and consequently incapable of 

 serving as a link, the two carbon atoms must be directly united, 

 and their valencies must grasp the hydrogen atoms, and the free 

 valency of the oxygen. The only structure which fulfils all the 



conditions, is 



H H 



I I 

 H— C — C— 0— H 



I I 

 H H 



which for convenience is represented by the constitutional formula 



C 2 H 5 • OH, or ethyl hydroxide. Acetylene consists of two atoms 



of hydrogen and two of carbon, the latter being united with three 



valencies since the hydrogen exhausts only one : thusH — C=C — H. 



When passed through a red-hot tube it polymerises, or condenses, 



into one-third the volume, forming benzene, C 6 H 6 . From the fact 



that the hydrogen atoms may be shewn to be really equivalent 



and can very readily be replaced by other monovalent atoms or 



molecules, while the carbon nucleus is of great stability ; that 



the molecule will suffer the addition of six halogen atoms 



and no more ; that only one mono-substitution product, and not 



more than three di-substitution products can be formed from it, 



and from a number of other considerations of an analogous nature, 



it is evident that benzene is structurally represented by a hexagon 



or ring of six carbon atoms, with the six hydrogen atoms on the 



outside, thus 1 : — 



1 It will be noticed that one valency of the quadrivalent carbon has 

 not been utilized. This enables benzene to take up six monovalent atoms 

 as in benzene hexahydride, or hexamethylene. The free valencies have 

 been supposed by Armstrong [1887] to be exercised in increasing the 

 stability of the^nucleus. 



C— May 3, 1899. 



