B.—CHEMISTRY 31 
of the inert gases. ‘These gases owe their inertness to the extreme stability 
of the ‘closed shells’ of electrons represented by the terms of the 
Rydberg series. ‘These shells are, indeed, so stable that the elements are 
devoid of all ordinary chemical properties, although under the stress of 
great excitement pairs of atoms can be wedded into diatomic molecules. 
From the Rydberg series, the electronic theory of valency emerges at 
once, since maxima of chemical reactivity are found in those metals which 
can acquire the electronic configuration of an inert gas by parting with 
one or two surplus electrons, and in non-metals which have a like deficit 
in their electronic budget. Inorganic chemistry, which consists so largely 
of the chemistry of ions, thus finds a firm foundation in the Thomson- 
Kossell conception of ‘ electron transfer’ between the atoms of unlike 
elements. On the other hand, the bonds by which atoms of similar 
elements are united in diatomic gases, and in the complex molecules of 
organic compounds, can be expressed by means of the Thomson-Lewis 
conception of ‘ shared electrons,’ for which a physical interpretation has 
now been found in the spinning electron of the older quantum mechanics, 
and the resonance energy of the later wave-theory. 
CHEMICAL CHANGES IN THE NUCLEUS. 
If the study of the electronic atmosphere is of primary value to the 
chemist in his studies of chemical reactions, it is impossible to deny that 
the study of the structure of the nucleus itself is of even more funda- 
mental significance, since it is here that the atomic numbers have their 
origin ; and, if it were not for the stability of certain selected nuclear 
structures, the chemist would have no atoms from which to construct 
his molecules, except perhaps the ultimate elements (apparently once 
more four in number) from which the nuclei are built. 1 need not now 
describe in detail the chemical interest which attaches to the discovery of 
isotopes, since this will form the basis of a subsequent discussion ; but I 
should like tomention Oliphant’s (4) separation of the isotopes of lithium, 
in sufficient quantities to test their behaviour towards high-speed protons 
and deutons, by the method of the mass-spectrograph, since this method 
is obviously capable of universal application, when developed on an ade- 
quate scale of magnitude. On the other hand, attention may be directed 
to the vast field of nuclear chemistry which has been opened up in recent 
years by the development of new projectiles for bombarding the nucleus. 
Thus the relatively clumsy «-particle, with its double positive charge, 
has been supplemented by the swift proton and deuton, with only a 
single positive charge to impede their approach to the positively charged 
target ; and a climax has been reached by using the neutron, which can 
approach the nucleus without impedance by any electric charge, like 
aircraft attacking a battleship. It can therefore score direct hits, which 
are found to have a devastating effect even on the stoutest nuclei. As a 
result of the introduction of this new projectile, no element can now be 
regarded as safe from disintegration ; and isotopes of short life promise 
in the future to become as common amongst the lighter elements as they 
are now amongst the spontaneously radioactive elements, which lie on 
the heavy side of the boundary formed by metallic lead. 
