26 SECTIONAL ADDRESSES 
through pipeclay, though extremely tedious, had more success and I was 
able to announce at the meeting of the Association at Birmingham in 
1913 that, after thousands of operations, a definite change of density, 
amounting to about 0-7 per cent., had been achieved. Further data 
from positive rays was obtained, and, when the War stopped work, there 
were several lines of reasoning indicating that neon consisted of two 
bodies of different mass, and that the behaviour of these was exactly 
that predicted by Soddy for isotopes, but none of these was sufficiently 
strong to carry conviction on so important a conclusion. 
During the War Soddy’s prediction concerning the atomic weights of 
leads from uranium and thorium minerals had been triumphantly vindi- 
cated by some of his most severe critics, the experts in chemical atomic 
weights, and when work was started again, although I continued for a time 
to experiment on separation by diffusion by means of an automatic 
apparatus, I realised that the most satisfactory proof of the existence of 
isotopes among the elements in general was only to be obtained by much 
more accurate analysis of positive rays. This was done by means of 
a sequence of electric and magnetic fields which gave focussed images of 
fine collimating slits, thus forming a spectrum dependent upon mass alone. 
This I called a ‘ mass-spectrograph’ (see footnote 1). It had a 
resolving power of about 1 in 130 and an accuracy of mass measurement 
of 1 in 1,000. This was ample to prove in 1919 that neon consisted, 
beyond doubt, of isotopes 20 and 22, and that its atomic weight 20-2 was 
the result of these being present in the ratio of about g to 1. Chlorine 
was found to contain 35 and 37, and bromine, of atomic weight almost 
exactly 80, and hence expected to be simple, gave two equally intense lines 
79 and 81. Other elements were shown to be much more complex. 
Krypton, the first of these, had six isotopes, 78, 80, 82, 83, 84, 86; xenon 
and tin even more. Of the greatest theoretical importance was the fact 
that the weights of the atoms of all the elements measured, with the 
exception of hydrogen, were whole numbers to the accuracy of measure- 
ment. This ‘whole number rule’ enabled the simple view to be taken 
that atoms were built of two units, protons and electrons, all the former 
and about half the latter being bound together to form the nucleus. 
Although the interpretation of mass-spectra was often far from simple 
owing to the difficulty of distinguishing between lines due to compound 
molecules and those representing true atomic mass-numbers the analysis 
of the more suitable elements advanced rapidly. Dempster at Chicago 
discovered the isotopes of magnesium, calcium, and zinc by means of an 
instrument of his own design with semi-circular magnetic focussing. By 
1925, when I replaced my first mass-spectograph, now in the Science 
Museum, South Kensington, with one of higher resolving power, informa- 
tion on the isotopic constitution of more than half the elements had 
already been obtained. ‘The new instrument was designed primarily for 
measuring the minute variations of the masses of atoms from the whole 
number rule, and had a resolving power ample for the heaviest elements. 
By its means the search for isotopes has been carried on until a few 
months ago. 
The difficulty of obtaining the necessary rays for analysis varies 
enormously from element to element. ‘Two main devices are employed : 
