446 Dr. J. P. V. Madsen on 



been assumed that the incidence 7 radiation from all the 

 substances experimented with is homogeneous. If, for ex- 

 ample, the incidence radiation from Pb contained a very soft 

 bundle in addition to the hard, many of the results obtained 

 by Kleeman could be immediately explained. With the 

 present form of apparatus I have not of course been able to 

 test this point ; it does not, however, seem at all improbable 

 that Pb should give out an exceedingly soft bundle of 7 rays, 

 the product of the soft 7 rays in the original radiation ; 

 indeed, if it is safe to make comparisons from the behaviour 

 of such substances as Zn, it would be surprising if such a 

 soft bundle did not exist, although of course its effect may 

 be small. Thirdly, in dealing with the very soft 7 rays, 

 which constitute the secondary incidence radiation, it seems 

 necessary to take into account the effect of tertiary radiation. 



In conclusion it may not be out of place to discuss one or 

 two points of some interest which have arisen during the 

 progress of these experiments. 



In fig. 8 it will be observed that for a mass of absorbing- 

 screen 70 the result of cutting out a considerable amount of 

 the secondary radiation has been to change the value of the 

 effect produced with Zn screens from 56 to 43. In the case 

 of Pb the corresponding change is from 38 to 30. In other 

 words, the ratio of the effect produced by a Zn screen to 

 that produced by a Pb screen of the same mass is apparently 

 much the same whether secondary radiation is included or 

 not. It appears almost as if the effect produced in the ioni- 

 zation chamber were due more to the secondaries produced 

 from the hard 7 rays than to direct effects of these hard 7 

 rays. The hard 7 rays are not apparently broken up as the 

 result of the first collision ; a few may be, but most of the 

 rays seem capable of passing into what we may for conveni- 

 ence speak of as an intermediate stage. A second collision 

 when the ray is in this stage may be more effective in 

 breaking up the 7 ray and causing the /3 particle to be sent 

 out. In some cases the 7 ray may still retain the character- 

 istics of the 7 ray even after many collisions. 



It w 7 ould seem possible for a 7 ray to produce some ioni- 

 zation in a gas as a result of its passage through that gas, 

 slow-speed delta rays being produced from the atoms of the 

 gas. Where high-speed electrons appear they are to be 

 considered as having been produced from the 7 ray itself, 

 being originally a part of it, as already suggested by Pro- 

 fessor Bragg. 



It has been found by Eve (Phil. Mag. 1906) that the 7 



