PRESIDENTIAL ADDRESS. 439 
functional activity of the hydrocarbon benzene and its immediate deriva- 
tives ; the attempts which have been made of late years to resuscitate the 
Kekulé oscillation hypothesis in one form or another appear to me to be 
devoid of practical significance. Any formula which represents benzene as 
an ethenoid must be regarded as contrary to fact. But in considering the 
properties of benzenoid compounds generally, it is necessary to make use of 
the Kekulé conception as well as the centric expression. The model of 
benzene devised by Barlow and Pope subserves a somewhat different and 
complementary purpose, being primarily of importance on the geometric 
side in discussing the relation of form to structure.* 
The discovery of trimethylene by Freund and the subsequent introduc- 
tion of synthetical methods of preparing polymethylenes by W. H. Perkin, 
jun., mark the onset of a new era, opening out as they did the possibility of 
understanding the structure of camphor and the terpenes and other con- 
stituents of the volatile oils from plants. 
Chemist after chemist had attempted in vain to solve the riddle pre- 
sented by camphor. Suddenly, in a moment of inspiration, a satisfactory 
solution of the problem was offered by Bredt. The acceptance of the bridged 
ring, the special feature of the Bredt formula of camphor, marks the intro- 
duction of a new moment into organic chemistry. 
The recognition of similar rings in several hydrocarbons of the terpene 
class, mainly in consequence of the masterly work of yon Baeyer, has con- 
tributed in no slight degree to an understanding of these compounds ; 
nevertheless, much remains to be learnt and there are many and serious 
difficulties to be overcome before we shall be in a position to appreciate the 
genetic relationship of all the substances included in the group. When the 
account of the work is written it will form one of the most striking and 
fascinating chapters in the history of our science. 
Among the many names of those who have contributed to its development 
the first to be mentioned is that of Wallach, to whose unwearied efforts, 
* The time is now approaching when it will be possible to extend the study of 
benzenoid compounds beyond the formal and superficial stage; hitherto we have 
been content to develop the methods of preparing such substances and to determine 
their number and their distinctive properties. Everything has to be learnt as to 
the exact character of the changes which attend their formation from the parent 
substance benzene and as to the exact nature of their inter-relationship. The 
impression produced by benzene, in my mind, is that of an eminently plastic 
system capable of responding to every slight change that may be impressed upon 
it. Nothing is more remarkable than the difference between benzene and its 
homologues, co obvious in the extraordinary increase in activity which attends the 
introduction of hydrocarbon radicles in place of one or more hydrogen atoms. But 
such plasticity is not characteristic of benzene only : if the properties of benzene- 
sulphonic acid be contrasted with those of the various substituted sulphonic acids, 
it is clear that every variation in the nucleus meets with some response from the 
sulphonic group ; what is still more remarkable, if the hydrogen in the hydroxylic 
group in the phenolsulphonic acids be displaced by other radicles, not only does the 
oxygen atom to which the radicle is attached seem to respond to the change but 
the benzenoid system and the still more distant sulphonic system are also 
both affected. It is well known that the physical constants are all variables in the 
case of benzenoid compounds. Perhaps the most remarkable confirmation of the 
view here advanced, however, is that afforded by the conclusion arrived at by 
Barlow and Pope that in the case of benzene derivatives, although the spheres of 
influence of the carbon and hydrogen atoms are relatively the same as in the 
parent compound, the spatial arrangement of the component spheres of atomic 
influence remaining practically unchanged, nevertheless the actual] volumes of the 
spheres of influence of both carbon and hydrogen alter proportionally to the altera- 
tion in molecular volume. Thus they maintain that in the cage of the conversion 
of benzene (molecular volume 77:4) into tetrabromobenzene (molecular volume 
130-2), the volumes of the spheres of influence of both carbon and hydrogen expand 
_jn the ratio of 77°4: 130°2, Such a conclusion is yery noteworthy, 
