20 THE PRESIDENTIAL ADDRESS. 
by the Periodic Law of Mendeléeff, or the Law of Octaves of 
Newlands. , 
The valency of the elements, like their periodicity, is a consequence 
of the principle that equilibrium becomes unstable when there are more 
than eight electrons in the outer layer of the atom. For on this view 
the chemical combination between two atoms, A and B, consists in the 
electrons of A getting linked up with those of B. Consider an atom 
like that of neon, which has already eight electrons in its outer layer ; 
it cannot find room for any more, so that no atoms can be linked to it, 
and thus it cannot form any compounds. Now take an atom of fluorine, 
which has seven electrons in its outer layer; it can find room for one, 
but only one, electron, so that it can unite with one, but not with more 
than one, atom of an element like hydrogen, which has one electron 
in the outer layer. Fluorine, accordingly, is monovalent. The oxygen 
atom has six electrons; it has, therefore, room for two more, and so can 
link up with two atoms of hydrogen: hence oxygen is divalent. Simi- 
larly nitrogen, which has five electrons and three vacant places, will 
be trivalent, and so on. On this view an element should have two 
valencies, the sum of the two being equal to eight. Thus, to take 
oxygen as an example, it has only two vacant places, and so can only 
find room for the electrons of two atoms; it has, however, six electrons 
available for filling up the vacant places in other atoms, and as there 
is only one vacancy to be filled in a fluorine atom the electrons in an 
oxygen atom could fill up the vacancies in six fluorine atoms, and 
thereby attach these atoms to it. A fluoride of oxygen of this compo- 
sition remains to be discovered, but its analogue, SF,, first made known 
by Moissan, is a compound of this type. The existence of two valencies 
for an element is in accordance with views put forward some time ago 
by Abegg and Bédlander. Professor Lewis and Mr. Irving Longmuir 
have developed, with great ingenuity and success, the consequences 
which follow from the hypothesis that an octet of electrons surrounds 
the atoms in chemical compounds. 
The term ‘ atomic weight ’ has thus acquired for the chemist an alto- 
gether new and much wider significance. It has long been recognised 
that it has a far deeper import than as a constant useful in chemical 
arithmetic. For the ordinary purposes of quantitative analysis, of tech- 
nology, and of trade, these constants may be said to be now known with 
Supreme importance. Their determination and study must now be 
of the essential nature of matter and on the ‘ superlatively grand ques- 
tion, What is the inner mechanism of the atom?’ they become of 
supreme importance Their determination and study must now be 
approached from entirely new standpoints and by the conjoint action of 
chemists and physicists. The existence of isotopes has enormously 
widened the horizon. At first sight it would appear that we should 
