180 MecCiLEeLtanp— On Secondary Radiation. 
and A’ show the importance of the part played by the secondary rays, and how 
they may influence the value calculated for the coefficient of absorption. 
Comparing curves A’ and B, in both of which the foil is placed close to the 
ionisation tube, the secondary rays appear to be more easily absorbed than the 
primary ; but the conditions of the experiments are not quite the same in the two 
cases, as the secondary rays are not confined to a narrow pencil as in the case of 
the primary, and therefore the relative position of the absorbing tinfoil is not so 
important. The difference in penetrating power is probably, therefore, not as 
great as a comparison of the curves A’ and B would suggest. 
It will be observed that the primary curves A and A’ become practically 
horizontal, which is, of course, due to the presence in the primary pencil of the 
very penetrating y rays. The secondary curve B, on the other hand, continues to 
descend with increasing thicknesses of foil; this is because the secondary pencil 
consists entirely of 8 rays. The magnetic experiments showed that it was possible 
to deflect all the secondary rays. The experiments therefore did not detect any y 
rays produced by the stoppage of the primary £ particles, or by the starting of the 
secondary 8 particles; such rays may have existed in small amounts and escaped 
detection, as we must remember that, in every case, a correction has to be made 
for the small normal ionisation in the tube 7. 
The importance of the secondary rays in the above experiments suggests a more 
detailed consideration of the manner in which matter is penetrated by a stream of 
charged particles like 8 rays; whether, in fact, there is great penetration by the 
primary particles, or whether it is not a successive stopping of one set and starting 
of another set of particles. The secondary rays, from the above experiments, are 
not very different in penetrating power from the rays that produce them, and the 
magnetic experiments described below indicate that the secondary rays contain 
some even more penetrating than the primary. The point is under further 
consideration. 
