Hacxert—The Secondary Radiation excited by y Rays. 213 
TaseE II. 
| az eeaE 
Element aA | fix | p | AB/p Agip(1 + / 1 — Kp) 
| 
Lead, 91 -339 11°4 8-0 10:7 
Platinum, . 175 339 21°5 8:0 10:9 
Tin, . 54 *406 7:3 7:4 10°4 
Cadmium, . 66 412 8°6 5) 10:6 
Silver, 85 -418 10°5 8-0 10:3 
Zinc, 48 464 7:2 66 9°6 
Copper, . 50 469 8-9 5°6 8:3 
Nickel, 0 63 *495 8°5 76 11°3 
Aluminium, 14 612 PRU 5:0 8:2 
But the relative secondary radiation is practically unchanged as we ascend 
the elements in the table of atomic weights from aluminium to tin, so that o/M 
must be almost constant for atomic weights ranging from 27 to 120, but increases 
very rapidly for lead and platinum. ‘These results may therefore be summed up 
in the broad general statement that the secondary radiation per atom (c) 
excited by y rays is roughly proportional to the atomic weight when this does 
not exceed 120, but increases more rapidly than the atomic weight for elements 
like lead, platinum, and uranium. J. J. Thomson has advanced the hypothesis 
that the number of electrons in the atom is equal to or of the same order as the 
atomic weight. The above result accords with this view if we suppose that, for 
the smaller atomic weights, the chance of the expulsion of an electron from an 
atom, whose stability is disturbed by a penetrating type of rays such as y rays, is 
independent of the atomic grouping, and proportional simply to the number of 
electrons in the atom. 
If the substance were in the form of a gas, then the ionisation in the gas 
would be due in part to this penetrating secondary radiation (o), and to the 
ionisation produced by 8 particles so expelled from the atom, and in part to any 
other type of secondary radiation (o’) which may exist, but which is easily 
absorbed, and so is not measured in the foregoing method. The second part will 
be in a constant ratio to the first, if the ionising power of the secondary B rays 
does not vary for different substances. The results at the end of this paper show 
that they have the same order of penetrating power, and so can be assumed to 
have the same ionising power. In this case, the ionisation per unit volume would 
be proportional to o (1 + C@) + o’ where C is a constant. Kleeman* has 
determined the ionisation produced in gases by y rays, and has found it to be 
an additive quantity for the atoms in the molecule. He has thus been able to 
deduce the relative ionisation per atom. His numbers are given below. 
* R. D. Kleeman, Proc. Roy. Soc., A vol. lxxix., p. 220, 1907. 
