Natures of various Electric Radiations. 431 



same 8 particles. The same author has shown by a direct 

 comparison that the velocity of these particles is the same as 

 that of the 8 rays displaced by cathode rays, i. e., about 

 3*3 X 10 8 cm./sec, or the velocity due to about 20 volts, a 

 velocity only slightly larger than that found by Lenard. 



As regards /3 and y rays, it is true that it has not been 

 definitely proved that most of the ionization which thev cause 

 is of the 8 type. But this may be inferred from well-known 

 experiments, such as those of Durack (PhiL Mag., May 1903), 

 or McClelland (Trans. Roy. Dub. $oc, February 1906). 

 When a pencil of ft radiation is allowed to cross an ioniza- 

 tion-chamber normally, and fall upon the opposite wall, it 

 gives rise to a secondary ionization, less in quantity, but not 

 much less in speed than the primary. A tertiary radiation 

 is caused by the secondary rays if they impinge on the 

 walls of the chamber, and there will doubtless be still further 

 derivations. But it appears that the quantity of the derived 

 radiations dies away much more quickly than the speed. 

 Thus the chamber is crossed and re-crossed (a few times) by 

 electrons of high speed, able to traverse an average path of 

 about 100 cm. in air at atmospheric pressure. If the 

 chamber is first exhausted and air gradually admitted, it is 

 found that the number of ions produced by the /5 rays is 

 proportional to the pressure. The paths of the ft rays will 

 not be appreciably affected by the introduction of the air ; 

 and so the experimental results are consistent with the simple 

 hypothesis that the ft particle (primary or secondary) makes 

 slow-speed ions in proportion to the number of gas atoms 

 traversed. Nor does any other hypothesis seem to be con- 

 sistent with the facts. It cannot be supposed that the bulk 

 of the ionization which is caused in the ionization-chamber 

 consists of high-speed secondary rays, though, of course, 

 these are originated when the primary rays strike the metal 

 surface of the chamber, and to a small extent when thev 

 strike gas molecules. For if all the negative electrons set 

 free by the ft rays were of high velocity we should expect 

 certain effects, as may be seen from the following considera- 

 tions, and none of these effects have been observed. 



Rutherford has shown (' Radioactivity/ 2nd edition, p. 431) 

 that the a particle of Ra makes about 86,000 ions in air ; 

 that one ft particle is emitted from Ra for every four cc par- 

 ticles ; and that the ionization due to ft particles is of the 

 order of 1 per cent, of that due to a particles in the case of 

 Ra in equilibrium. Thus the ft particle of Ra produces 

 some thousands of ions. This is also evident from the ex- 

 periments of Durack (PhiL Mag., May 1903\ who has shown 



2 a 2 " 



