30 SECTIONAL ADDRESSES 
exact experiment in his life ’(1)* ; and the physico-chemical theories which 
first attracted me to the study of chemistry were largely fallacious, since 
we now know that the concentration of ions in an aqueous solution cannot 
be deduced directly from its conductivity at different dilutions ; nor does 
the catalytic activity of an acid afford a direct measure of the concentra- 
tion of hydrogen ions which it contains, in view of the fact that the 
molecules of the acid may be even more active than the ions produced 
from them. Even more amazing evidence of inaccurate theory was the 
claim made by Ostwald in 1904 (2) that the law of multiple proportions 
(which Sommerfeld (3) cites as one of three main arguments for the atomic 
theory) could be deduced without the help of the atomic hypothesis! At 
the present time, however, the work of Dr. Aston in the Cavendish 
Laboratory, and of Professor Lennard-Jones in the Chemical Laboratory 
at Cambridge, may be cited as a proof of interpenetration, which is as 
welcome as it is undoubtedly beneficial to both laboratories. Moreover, 
if I may be allowed to make a more personal remark, the efficiency of my 
own Laboratory of Physical Chemistry at Cambridge, and the pleasure 
that I derive from directing it, depend largely on the fact that the workers 
in the laboratory consist of chemists and physicists in approximately 
equal numbers, so that we are equally well equipped for work in the 
older Physical Chemistry and in the newer Chemical Physics. Indeed, 
our chief need at the present time is for larger numbers of organic 
chemists to undertake researches in the physical chemistry of organic 
compounds, which do not necessarily require (as is so often feared) a 
knowledge of wave mechanics and a mastery of higher mathematics. 
Atomic NUMBERS. 
If I were asked to indicate the principal contribution which physics 
has made to the progress of chemistry during the present century, I 
should without hesitation point to the theory of atomic numbers, and to 
the galaxy of phenomena that are associated with it. We might begin, 
for instance, by defining the atomic number of an atom as the net positive 
charge of the nucleus, on the assumption that Rutherford’s ‘ nucleus 
atom ’ is too stable to be disintegrated by any verbal bombardment to 
which it may be submitted. We then pass immediately to the epoch- 
making conclusion that nuclear charge is more important to the chemist 
than atomic mass, since the chemical properties of an element depend 
almost exclusively on the configuration of the electronic atmosphere with 
which the nucleus envelops itself in the neutral atom or in the ions derived 
from it. 
When the atomic numbers of the elements were made known, through 
the experiments of Moseley and others, a precise numerical basis was 
provided for their periodic classification. ‘This finds its simplest ex- 
pression in the Rydberg series : 
2X i? +2 XxX 22-2 X27 +2 %'32-+2 3% +2 4b ge 
which tells us how many electrons are required to give the configuration 
* References will be found at the end of the Address. 
