B.—CHEMISTRY. 33 
For instance, Kekulé said in 1867 : 
* The question whether atoms exist or not has but little significance 
_ from a chemical point of view; its discussion belongs rather to philo- 
sophy. In chemistry we have only to decide whether the assumption 
of atoms is an hypothesis adapted to the explanation of chemical 
phenomena . . . and to advance our knowledge of the mechanism of 
chemical phenomena ’ 
“Horas to explain known facts and, above all, to predict new facts. 
and it is probable that throughout the nineteenth century it was a matter 
of comparative indifference to most scientific chemists whether atoms had 
a real existence. All that was important was that matter behaved as 
though it had an atomic structure, and that no fallacies or errors were 
introduced by making such an assumption. The value of the atom to 
them was quite independent of any possible demonstration of its real 
existence. Gradually, as the conception of atoms and molecules was 
found to fit a larger and larger field of facts, confidence grew, and atoms 
came to be regarded as real, in the only sense in which the scientific experi- 
menter can understand reality. Molecules, built up of atoms according 
to well-established laws, shared in this confidence, which was thoroughly 
_ justified by the remarkable concordance of the determinations of Avo- 
gadro’s number, the number of molecules in the gramme molecule of 
a substance, as arrived at by a number of totally independent methods. 
_ The discovery of radio-activity, whilst enlarging the conception of the 
atom, has made it possible to isolate the effects of single atoms travelling 
at a high velocity, so that the impact of a single «-ray on a fluorescent 
sereen produces a visible effect, and the counting of these rays, which 
are known to be charged helium atoms, corresponds perfectly with the 
original hypothesis. When the minuteness of the atom was realised, 
chemists cannot have dreamed that a day would come when the effect 
of so extraordinarily minute a particle could be perceived by the eye and 
even exhibited on a screen to an audience. No more extraordinary 
confirmation of the soundness of the theoretical views of the chemists 
of the early nineteenth century could have been received. 
It is strange to remember that little more than twenty years ago it 
was proposed by Franz Wald, and the idea was adopted by some chemists, 
that the atomic conception might be dispensed with in chemistry, and 
that the science might advance by making use of thermodynamical 
conceptions alone. It is certain that such a proposal could not have 
been made by an organic chemist, to whom reasoning on structural lines 
is habitual. It has been said that the establishment of the structural 
formula of an organic compound of some complexity, such as an alkaloid 
or a triphenylmethane dye, by successive, carefully chosen steps of 
analysis and synthesis, is the best illustration of the principles of scientific 
Teasoning, and there is much truth in the contention. Chemists, there- 
_ fore, were not inclined to follow so illusory a path, and the proposal has 
met with no acceptance. The later development of chemistry has been 
entirely in the opposite direction, that of leaning with greater and greater 
confidence on the atomic and molecular foundations of the science. 
3! Next came the development of structural theory, with reference to 
organic compounds, associated with the names of Kekulé, Couper, Crum 
— -1925 D 
