PRESIDENTIAL ADDRESS, 32D 
Physics of the last century as the mathematical theory of light and the kinetic 
theory of gases are monuments to the powers of the human intellect. 
The path of Chemistry, as an application of pure logical argument to the 
interpretation of complex masses of observations, thus gradually diverged from 
that taken by Physics as the mathematical treatment of less involved experi- 
mental data, although both subjects derived their impetus to development from 
the speculations of genius. 
It is interesting to note, however, that during recent years the two sciences, 
which were so sharply distinguished twenty years ago as to lead to mutual 
misunderstandings, are now converging. Many purely chemical questions have 
received such full quantitative study that the results are susceptible to attack 
by the methods of the mathematical physicist; on the other hand, the intense 
complication perceived during the fuller examination of many physical problems 
has led to their interpretation by the logical argument of the chemist because 
the traditional mathematical mode of attack of the physicist has proved power- 
less to deal with the intricacies exhibited by the observed facts. 
The progress of Chemistry during the last century has been mainly the result 
of the co-ordination of observed facts in accordance with a series of hypotheses 
each closely related in point of time to the one preceding it. The atomic theory, 
as it was enunciated by Dalton in 1803, was a great impetus to chemical investi- 
gation, but proved insufficient to embrace all the known facts; it was supple- 
mented in 1813 by Avogadro’s theorem—that equal volumes of gases contain the 
same numbers of molecules at the same temperature and pressure. These two 
important theoretical developments led to the association of a definite physical 
meaning with the idea of molecular composition, but ultimately proved 
insufficient for the interpretation of the ever-increasing mass of chemical know- 
ledge collected under their stimulus. A further great impetus followed the 
introduction by Frankland and Kekulé, in 1852 onwards, of the idea of valency 
and the mode of building up constitutional formule; the conception of molecular 
constitution thus arose as a refinement on the Daltonian notion of molecular 
composition. In course of time the theoretical scheme once more proved 
insufficient to accommodate the accumulated facts, until, in 1874, van ’t Hoff 
and Le Bel demonstrated the all-important part which molecular configuration 
plays in the interpretation of certain classes of phenomena known to the 
organic Chemist. 
During the early days of chemical science—those of Dalton’s time and perhaps 
also those of Frankland and Kekulé—we can believe that chemical theory may 
have lacked the physical reality which it now seems to us to present; the 
attitude of our predecessors towards the theoretical interpretation of their 
observations was rather that described by Plato: ‘as when men in a dark 
cavern judge of external objects by the shadows which they cast into the 
cavern.’ In the writings of the most clear-sighted of our forerunners we can 
detect an underlying suspicion of a possibility that, at some time or other, the 
theory by means of which chemical observations are held together may undergo 
an entire reconstruction; a very few years ago Ostwald made a determined 
attempt to treat our science without the aid of the molecular hypothesis, and 
indeed suggested the desirability of giving the Daltonian atomic theory decent 
burial. 
The last ten years or so has seen a change in this attitude. The development 
of Organic Chemistry has revealed so complete a correspondence between the 
indications of the conception of molecular constitution and configuration and 
the observed facts, and recent work on the existence of the molecule, largely 
in connection with colloids, with radioactivity, and with crystal structure, is so 
free from ambiguity, that persistence of doubt seems unreasonable. Probably 
most chemists are prepared to regard the present doctrine of chemical consti- 
tution and configuration as proven; although they may turn a dim vision 
towards the next great development, they have few misgivings as to the stability 
of the position which has already been attained. 
Let us consider how far the study of Organic Chemistry has hitherto led us; 
we may pass over the gigantic achievements of those who in past generations 
determined constitution and performed syntheses, thus making the subject 
one of the most perfect examples of scientific classification which exist, and turn 
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