18 THE PRESIDENTIAL ADDRESS. 
distillation at very low pressures, and have been shown to vary very 
slightly in density. Lithium is found to consist of two isotopes, 
6 and 7. Sodium is simple, potassium and rubidium are complex, each 
of the two latter elements consisting, apparently, of two isotopes. The 
accepted atomic weight of cesium, 132.81, would indicate complexity, 
but the mass spectrum shows only one line at 133. Should this be 
confirmed cesium would afford an excellent test case. The accepted 
value for the atomic weight is sufficiently far removed from a whole 
number to render further investigation desirable. 
This imperfect summary of Mr. Aston’s work is mainly based upon 
the account he recently gave to the Chemical Society. At the close 
of his lecture he pointed out the significance of the results in relation 
to the Periodic Law. It is clear that the order of the chemical or 
‘mean’ atomic weights in the Periodic table has no practical signifi- 
cance; anomalous cases such as argon and potassium are simply due to 
the relative proportions of their heavier and lighter isotopes. This 
does not necessarily invalidate or even weaken the Periodic Law which 
still remains the expression of a great natural truth. That the expres- 
sion as Mendeléeff left it is imperfect has long been recognised. The 
new light we have now gained has gone far to clear up much that was 
anomalous, especially Moseley’s discovery that the real sequence is the 
atomic number, not the atomic weight. This is one more illustration 
of the fact that science advances by additions to its beliefs rather than 
by fundamental or revolutionary changes in them. 
The bearing of the electronic theory of matter, too, on Prout’s dis- 
carded hypothesis that the atoms of all elements were themselves built 
up of a primordial atom—his protyle which he regarded as probably 
identical with hydrogen—is too obvious to need pointing out. In a 
sense Prout’s hypothesis may be said to be now re-established, but 
with this essential modification—the primordial atoms he imagined are 
complex and are of two kinds—atoms of positive and negative elec- 
tricity—respectively known as protons and electrons. These, in Mr. 
Aston’s words, are the standard bricks that Nature employs in her 
operations of element building. 
The true value of any theory consists in its comprehensiveness and 
sufficiency. As applied to chemistry, this theory of ‘the inner 
mechanism of the atom’ must explain all its phenomena. We owe to 
Sir J. J. Thomson its extension to the explanation of the Periodic Law, 
the atomic number of an element, and of that varying power of chemical 
combination in an element we term valency. This explanation I give 
substantially in his own words. The number of electrons in an atom 
of the different elements has now been determined, and has been found 
to be equal to the atomic number of the element, that is to the position 
which the element occupies in the series when the elements are 
