y 
f 
6 McCrettanp—On Secondary Radiation and Atomic Structure. 
the slope of the curve at these parts. The flattening begins at or near the top of 
each period, and when we get into the next period, a more rapid increase of 
secondary radiation again sets in. 
Several of the periods are very incomplete—for example, a number of elements 
are missing below molybdenum ; and, as stated above, we have no representative 
of the third long period ; but still enough of the whole curve is drawn to show that 
there is a decided change of form going from one period to the next as compared 
with the form throughout any period. 
The flattening of the curve going from one period to another is a flattening of 
the top of the curve corresponding to one period rather than that of the lower 
portion of the curve corresponding to the next period. 
Again, although we have only one element in the first and last periods—carbon 
and uranium—still it is obvious that they give points on what are distinct portions 
of the curve, as in the case of the other periods. 
We shall now see how these results may possibly be explained. The prominent 
fact is that the secondary radiation increases as the atomic weight increases, and it 
is easy to picture in a general way how this result may be brought about. The 
incident primary radiation consists chiefly of 8 particles, each of which is small 
compared with an atom as a whole, so that we must regard each primary £ particle 
as colliding with or coming within the radius of action of an electron (or B particle) 
of the atom of the substance being tested for secondary radiation rather than 
with the atom as a whole. If, then, all atoms are composed of electrons, we shall 
have for a given incident primary radiation the same number of electrons 
disturbed in the substance tested, no matter what substance it is. The greater 
the atomic weight of the substance, the greater, therefore, is the disturbance 
produced in any one atom, and the greater the chance of detaching electrons 
to produce our secondary radiation. 
Then, in addition to the general result that the secondary radiation increases 
with the atomic weight, we have the division of the elements as shown in the 
table of results, and especially by the curves in fig. 2. 
We might describe the results shown by these curves by saying that as we 
pass through the members of a chemical period, the secondary radiation increases 
fairly rapidly as the atomic weight increases; but as we go from one period to 
the next, the increase of secondary radiation with atomic weight is much less 
rapid. When we get near the top of a period, there is very little increase of 
secondary radiation as the atomic weight increases; but when we get into the 
next period, the secondary radiation again begins to increase more rapidly. ‘The 
curve, if completed by the inclusion of the missing elements, would apparently 
have a flattened portion corresponding to the top of each period. Following the 
same line of explanation as before, this can be explained as follows :—Going from 
