of Radium, and Secondary Rays which they produce. <)61 
relation is not altered when a group of primary rays is 
transformed into a group of secondary rays. 
The difference in the absorption of a primary and secondary 
pulse (which, as we have seen, must be distinguished from 
selective absorption) is probably due in part or on the whole 
to the form of the force-relation of the secondary pulse satis- 
fying the condition for absorption better than the primary 
pulse. But there is probably some other cause operating 
besides this one. Now, since the coefficient of absorption of 
a 7 pulse is independent of the distance from its source, the 
difference in the absorption cannot be due to a difference in 
the force of the pulse. The difference in the absorption is 
therefore probably due in part to a difference in the breadth 
of the pulse, the pulses in a group of secondary rays being 
probably broader than those in the corresponding group of 
primary rays. 
The production of secondary rays and their properties may 
then be explained on general lines as follows. Each electron 
in an atom is in equilibrium under the action of the forces in 
the atom, and when displaced from its position of equilibrium 
by the force in a pulse passing over it, the atomic forces tend 
to bring it back to its previous position. The maximum 
absorption of energy from the pulse by the electron will take ' 
place if the force acting on the electron due to the electric 
force in the pulse is always proportional to the force of restitu- 
tion of the electron, whether both forces act in the same or in 
opposite directions. This is the condition, it will be observed, 
that acceleration of motion of the electron must take place all 
the time the pulse is acting upon it. That this condition 
may be satisfied depends on the form of the force-relation of 
the pulse as well as its breadth. The form of the force- 
relation of a pulse and its breadth, it may be pointed out in 
passing, may easily be such that at the instant the electron 
ceases to be under the influence of the pulse after having 
been deflected, it is at rest in its position of equilibrium. If 
a pulse passes over an electron which does not exactly satisfy 
the conditions for best absorption (we may suppose the pulse 
narrower for one thing than required by these conditions), 
the pulse radiated by the electron will satisfy these conditions 
much better, and will therefore be more easily absorbed by a 
similar electron than the primary pulse. If the pulse satisfies 
the conditions for best absorption to a small extent only, it 
w T ill be only slightly absorbed, and the secondary pulse will 
not be so soft in comparison with the primary as in the fore- 
going case. The selective absorption of a group of primary 
y rays by a substance may then be explained by the force- 
PhiL Mag. S. 6. Vol. 15. No. 89. May 1908. " 2 Y 
