SECTION A.—MATHEMATICAL AND PHYSICAL SCIENCES. 
THE STORY OF ISOTOPES 
ADDRESS BY 
FW ASTON, Se!D.YDSe.,' LED), FLOW RIS: 
PRESIDENT OF THE SECTION. 
Tuts chapter in the history of science contains much to interest the 
philosopher and offers many illustrations of that interplay of theory and 
experiment by which advance takes place. Theory is the scaffolding of 
science, and just as in ordinary building operations, though some parts of 
it may only be used for a short time before removal, others may function 
for so long a period that they may well be mistaken for the permanent 
structure itself. The postulate of Dalton (1803) that atoms of the same 
element are equal in weight is a good example of yery permanent scaffold- 
ing. For over a hundred years it was practically undisputed and on it 
was founded the major part of atomic chemistry. 
About ten years later Prout made the more speculative suggestion that 
all atoms were made up of primordial particles which he thought might 
be atoms of hydrogen. On this view the weights of all atoms must be 
expressed as whole numbers, and if, as Dalton postulated, the atoms of 
any particular element were all equal in weight, the atomic weights and 
combining ratios of all elements must be whole numbers also. Chemists 
soon found that this was certainly not in agreement with experiment ; 
the more results they obtained the more impossible it was to express the 
atomic weights of all the elements as whole numbers, and of the two 
theories Prout’s was the one to be abandoned. In this decision they were 
perfectly justified for, as it cannot be too often emphasised, it is more 
important for a scientific theory to be simple than for it to be true. Besides 
it was of little practical importance to chemists if atoms were not equal 
in weight so long as in all the ordinary operations of chemistry they 
behaved as though they were. 
Crookes, however, thought that he had found evidence that they did 
not so behave, and in his remarkable Presidential Address to Section B, 
at Birmingham in 1886, he says: ‘I conceive, therefore, that when we 
say the atomic weight of, for instance, calcium is 40, we really express 
the fact that, while the majority of calcium atoms have an actual atomic 
weight of 40, there are not a few which are represented by 39 or 41, a less 
_ number by 38 or 42, and so on.’ Later, he developed this idea in con- 
nection with his pioneer work on the rare earths. He called the com- 
ponents ‘ meta-elements,’ but unfortunately for his reputation as a prophet 
the experimental results on which his idea was founded were later proved 
to be fallacious, and Dalton’s postulate was reinstated as an article of 
scientific faith more firmly than ever. 
Its overthrow, deferred for another twenty years, was one of the many 
atastrophic results of the tremendous shock due to the discovery of 
