a Se Se ee ee 
THE PRESIDENTIAL ADDRESS. 19 
arranged in the order of their atomic weights. We know now the 
nature and quantity of the materials of which the atoms are made up. 
The properties of the atom will depend not only upon these factors but 
also upon the way in which the electrons are arranged in the atom. 
This arrangement will depend on the forees between the electrons 
themselves and also on those between the electrons and the positive 
charges or protons. One arrangement which naturally suggested itself 
is that the positive charges should be at the centre with the negative 
electrons around it on the surface of a sphere. Mathematical investi- 
gation shows that this is a possible arrangement 1f the electrons on the 
sphere are not too crowded. The mutual repulsion of the electrons 
resents overcrowding, and Sir J. J. Thomson has shown that when 
there are more than a certain number of electrons on the sphere, the 
attraction of a positive charge, limited as in the case of the atom in 
magnitude to the sum of the charges on the electrons, is not able to 
keep the electrons in stable equilibrium on the sphere, the layer of 
electrons explodes and a new arrangement is formed. The number of 
electrons which can be accommodated on the outer layer will depend 
upon the law of force between the positive charge and the electrons. 
Sir J. J. Thomson has shown that this number will be eight with a law 
of force of a simple type. 
To show the bearing of this result as affording an explanation of the 
Periodic Law, let us, to begin with, take the case of the atom of lithium, 
which is supposed to have one electron in the outer layer. As each 
element has one more free electron in its atom than its predecessor, 
glucinum, the element next in succession to lithium, will have two 
electrons in the outer layer of its atom, boron will have three, carbon 
four, nitrogen five, oxygen six, fluorine seven and neon eight. As there 
cannot be more than eight electrons in the outer layer, the additional 
electron in the atom of the next element, sodium, cannot find room in 
the same layer as the other electrons, but will go outside, and thus the 
atom of sodium, like that of lithium, will have one electron in its outer 
layer. The additional electron, in the atom of the next element, 
magnesium, will join this, and the atom of magnesium, like that of 
glucinum, will have two electrons in the outer layer. Again, alu- 
minium, like boron, will have three; silicon, like carbon, four; phos- 
phorus, like nitrogen, five; sulphur, like oxygen, six; chlorine, like 
fluorine, seven; and argon, like neon, eight. The sequence will then 
begin again. Thus the number of electrons, one, two, three, up to eight 
in the outer layer of the atom, will recur periodically as we proceed 
from one element to another in the order of their atomic weights, so 
that any property of an element which depends on the number of elec- 
trons in the outer layer of its atom will also recur periodically, which 
is precisely that remarkable property of the elements which is expressed 
