34 SECTIONAL ADDRESSES. 
Brown and Butlerow. Again the assumed arrangements of atoms in 
compounds were adopted in order to express the reactions of the 
substances, without reference to the real existence of the chains of atoms 
represented in the new formule. In 1867 Crum Brown wrote: “ While 
there can be no doubt that physical research points to a molecular constitu- 
tion of matter, it is perfectly indifferent to a chemist whether his symbols 
represent atoms or units; and graphical formule would be as useful as 
they are now, were it conclusively proved that matter is continuous.’ 
Within the last few years the study of the films of fatty acids and similar 
substances on the surface of water by Langmuir, Hardy and Adam has 
shown that the properties of such films can only be accounted for by 
assuming the reality of those chains of atoms which served so well the 
purposes of the chemist, but seemed physically improbable. The examina- 
tion of solid fatty acids by means of X-rays leads to exactly the same 
conclusion. The greatest triumph of structural theory, the hexagon 
formula for benzene, need only be mentioned in passing, since it is only 
a month or two since the celebration of the discovery of benzene by 
Faraday, when the wonderful chemistry of the aromatic compounds was 
eloquently described by Sir Wm. Pope and Prof. Armstrong. Next came 
the generalisation known as the periodic system of the elements, due 
mainly to Mendeleéff and to Lothar Meyer, and finally the hypothesis 
of the tetrahedral arrangement of the atoms around a carbon atom, 
devised by van t’Hoff and Le Bel to account for optical isomerism. Modern 
X-ray methods show that the structure of crystals of the corresponding 
substances is fully accounted for by assuming that the benzene hexagon 
and the tetrahedral linking of carbon are actually present, and the inter- 
pretation of crystals has been made possible and unambiguous by the 
existence of so great a mass of fully established chemical data. 
The point which I wish to make is that these hypotheses, of the 
chemical atom, of the molecule, of the chains and linkings represented in 
the graphic formule of organic compounds, of the hexagonal ring in 
aromatic substances, and of the tetrahedral carbon atom, were introduced 
without reference to any metaphysical conception of the nature of matter, 
and were independent of any dogma concerning reality; they were 
intended as working hypotheses, connecting and co-ordinating facts which 
had been discovered by the classical methods of chemical experimentation. 
That they have been confirmed by entirely independent physical means, 
so that they have become established as the truest representation we can 
have of nature, shows how sound was their foundation, and encourages 
us to suppose that the same methods which have served so well in the 
past may again ‘be trusted to lead to new discoveries in the future. The 
remaining example which I have mentioned, the periodic law, was regarded 
by many chemists as a convenient means of arranging the facts of inorganic 
chemistry, but was expressly stated to be only empirical, since a theoretical 
basis was inconceivable. The work of Moseley, the discovery of the 
radioactive elements, and the conception of isotopes, have shown the 
periodic classification to be the most fundamental thing in the chemistry 
of the elements, and the atomic number has been found to have greater | 
theoretical significance than the atomic weight. Reference to isotopes 
reminds us that this discovery also was made by chemical means, although 
its investigation appears almost to have passed out of the hands of the 
chemist into those of the physicist, since the introduction of the positive 
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