McCieLtanp— On Secondary Radiation and Atomic Structure. i 
one element to another of greater atomic weight in the same period, the additional 
electrons necessary to form the heavier atom are added on in a way that increases 
considerably the secondary radiation ; but in passing from one period to the next, 
the additional electrons are added on in some different way which is not so 
effective in producing an increase of secondary radiation. 
Again, we know that members of the same period have little in common 
chemically, but that each member of any period corresponds chemically to some 
members of the other periods. The electrons, therefore, which are added on 
as we go through a period, are those which determine the chemical properties, 
and are also those which are chiefly effective in increasing the secondary 
radiation. 
Let us further see how the chemical ‘‘ groups” are represented on our curves ; 
each chemical group is composed of elements having similar chemical properties. 
If we divide the curves corresponding to the long periods into two parts (as these 
periods are divided by the chemist into odd and even ‘‘series’’) at the places where 
the transitional elements occur, that is about iron, cobalt, and nickel in one case, 
slightly below silver in the second case, and at platinum in the third case, and 
regard each part as a separate curve, then we can say that each chemical group is 
made up of elements occupying similar positions on our curves, when completed 
so as to contain all the elements. The curves corresponding to short periods are, 
of course, to be taken as a whole. . 
Both the periods and the groups in the periodic divisions of the elements 
employed in chemistry are therefore represented in our curves in a very simple 
manner. 
Our results have therefore led to the view that the manner in which additional 
electrons are added on to form elements of greater atomic weight is different as 
we go from one period to the next, from what it is as we pass through elements not 
near the ends of a period. It will at once be seen that there is a close resemblance 
in this to the views advanced during recent years by J. J. Thomson as to the 
constitution of the atom (‘‘ Electricity and Matter,” and various papers). Professor 
Thomson deduces, from theoretical investigations on stability, that, as the number 
of electrons in the atoms are increased, they may take up a position on an outer 
shell, or they may go to form a central core, thus giving-in general two different 
ways of adding on the additional electrons. 
In conclusion, there are a few points in connexion with the numbers in the 
Table which may be noticed. 
The density of the substances is given in the fifth column, and we see that the 
density is of little importance in determining the secondary radiation. It might 
appear at first sight that the coefficient of absorption, which is approximately 
proportional to the density, should be an important factor, because it determines 
