294. TRANSACTIONS OF SECTION A, 
fact that appears from the scattering experiments is that the number of elec- 
trons (outside the nucleus) is about half the atomic weight. There is now 
fairly good evidence that, if the elements are numbered in order of atomic 
weight, the numbers will actually express the charge on the nucleus. The rate 
of vibration of the inner parts of the nucleus can now be measured by means 
of the characteristic X-rays emitted. Each substance has several strong lines 
in its X-ray spectrum, and as we pass from element to element in order of 
atomic weight the frequencies of these change by regular jumps. H. G. J. 
Moseley has investigated all the known elements in this way, and he is even 
able to show at what points elements are missing, because at such points the 
X-ray frequencies make a double jump. In this way he has found that between 
aluminium and gold only three elements are now missing. It is deduced from 
these considerations that there is something more fundamental in the atom than 
its atomic weight, viz., the charge on the nucleus, and that this is the main 
factor which controls the frequency of the interior vibrations, the mass having 
only a slight influence. 
There are certain elements with identical chemical properties, but different 
atomic weights. Thus Radium-B (atomic weight 214) and lead (207) are 
chemically inseparable and have the same y-ray spectrum. It is quite clear 
that some new conception is required to explain how the atoms, having the 
structure we have supposed, can hold together. N. Bohr has faced the diffi- 
culty by bringing in the idea of the quantum in a novel way. At all events, 
there is something going on in the atom which is inexplicable by the older 
mechanics, 
Professor Armstrone: Although chemists must admire as well as welcome 
the bold attempt physicists are making to unravel the structure of the 
elementary atom, they cannot yet with advantage discuss the conclusions 
arrived at by their colleagues; the arguments used are so novel and daring, the 
contentions so original, that at present they are not in a position to appreciate, 
still less to criticise them effectively; in fact, the chemist’s office at the 
moment must be mainly to point out the conditions that a theory must satisfy 
to meet his requirements. He has long been prepared to believe that the 
materials spoken of as elements may prove eventually to be compounds; indeed, 
the relationships between them are so similar to those manifest between carbon 
compounds, and of such a character, that it is almost necessary to believe in their 
composite nature; but the views that are now advocated by physicists are 
entirely different from any conceptions that chemists have ever entertained and 
cannot easily be assimilated by them. Physicists, unfortunately, in the past 
have held aloof from chemists; they have paid too little attention to their 
methods and to their results; the movement now in progress is therefore to be 
welcomed, as it must have the effect of leading the two parties in future to 
work together to a common end. Hence the value of the present discussion, 
It is doubtful if it be permissible at present to conclude that elements of 
different atomic weight may and do exist which are indistinguishable chemi- 
cally: the observations on which reliance is placed have been made with 
quantities of material far too small to permit of such an inference; in the case 
of the rare earth elements, although very large quantities of material have been 
at the disposal of chemists, they have only slowly discovered differences by 
which they are enabled to distinguish and separate them. Though the special 
methods made use of by physicists are very powerful, they suffice only in 
certain cases and have little chemical significance; when physicists resort to 
chemical methods the work becomes subject to ordinary criteria. 
The resemblance of the X-ray spectra of so many elements is undoubtedly 
most significant, but to conclude, on such evidence, that all but very few of 
the elements are discovered is scarcely justifiable; it may well be that most of 
those that are known belong to a certain ‘preferred’ type and that a particular 
series is nearly complete, the similarity of the spectra being perhaps due to the 
presence of a radicle common to the series, much as in the case of a series of 
related benzenoid compounds. In the case of carbon compounds, of the large 
number of series possible, it is well known that certain types are formed 
preferentially, being more stable or more readily produced than others. If the 
so-called elements are compound substances, it may well be that the occurrence 
