Hacxert—The Secondary Radiation excited by y Rays. 217 
method magnifies it slightly. On the above scale of intensity s= 5; and if we 
assume for lead f= ‘77, an extreme value, the difference made in taking /’= p is 
not 2 per cent., so that the method gives fairly approximate values for the 
absorption-coefticient. 
It has been shown theoretically that the absorption-coefficient of the 8 particle 
varies inversely as the fourth power of the velocity; and this result is borne out 
by direct experiment on 6 rays of different velocities. In the case of secondary 
rays we have probably a heterogeneous bundle of rays consisting of 8 particles 
travelling with different velocities, so that the absorption-coefticient can only 
indicate the average value of these velocities. Using this result, we can interpret 
Table IV. as showing that, with increasing hardness of the y rays, as increased 
penetrating power may conveniently be termed, the average velocity of the 
secondary rays increases markedly for elements of high atomic weight such as 
lead, not so much for elements of medium atomic weight such as copper, and does 
not vary at all for elements of low atomic weight such as aluminium. Tor very 
soit y rays, it decreases with increase of atomic weight; but for the penetrating 
y rays, it increases with the increase of atomic weight. 
Innes* has measured directly the velocity of the kathode particles emitted 
under the influence of Rontgen rays for elements of high atomic weight, such as 
lead, gold, and silver. He found that the velocity of the fastest increased with 
increasing hardness of the rays, and that the minimum velocity was nearly the 
same for all substances. His values for lead ranged from 6°3 to 7:3 x 10° cm. 
per sec. for soft rays, and from 6:3 to 8:3 x 10° cm. per sec. for hard rays. 
We cannot draw any conclusions concerning the minimum velocity from 
observations on the absorption-coefficients ; but it is clear that both in the case of 
Rontgen and y rays, the hardest rays cause the emission of the particles with the 
highest velocities. This is another similarity between the two types of rays. 
It is true, of course, that rays might become more penetrating by a proportionate 
increase in the number of the particles with high velocities. But even on this 
view, the hard rays cause the greatest emission of the fastest particles. 
The comparison between the secondary radiations excited by y and X rays 
cannot be carried very far, since, in the former case, the soft type of rays 
is certainly heterogeneous, and the hard rays have only been obtained by 
transmission through lead screens. It is not certain whether the hardening of 
the rays in this way is due to the absorption of the softer rays, or a transformation 
of the rays by passing through the substance. A more extended series of 
observations, using screens of different substances, is necessary before we make 
any wider generalizations or discuss the rival theories in the light of these results. 
* P. D. Innes, Proc. Roy. Soc., 1907. 
TRANS. ROY. DUB, S0C., N.S., VOL. IX., PART IX, 2M 
