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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-sp-eed 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 ionisation which is caused in the ionisation-chamber 

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

 are originated when the primary rays strike the metal sur- 

 face of the chamber, and to a small extent when they 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 considerations, and 

 none of these effects have been observed. 



Rutherford has shown (Radio-activity, 2nd edition, p. 

 434) that the a particle of Ra makes about 86,000 ions in 

 air: that one ft particle is emitted from Ra for every four 

 a particles ; and that the ionisation 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 that the ft particle produces about 130 ions 

 per cm. in air at atmospheric pressure. Now, the ft particle 

 runs a course in the open air of an average length of 100 

 cm. This leads to an estimate of its ionisation even greater 

 than that obtained by Rutherford. If all the electrons, so 

 liberated, had a high velocity, the energy set free would be 

 out of all proportion to that of the original ft particle. Yet 

 if we are to ascribe a high velocity to the electrons set 

 free, it must be a very higfh one, for it has been shown by 

 Allen (Phys. Review, August, 1906), that the sec- 

 ondary radiation of ft rays consists of electrons moving with 

 a speed approximating to that of the primary. We cannot 

 suppose that all these electrons are of this high-speed type. 

 Moreover, if this were the case, the free path of such elec- 

 trons would become comparable with the dimensions of the 

 ionisation-chamber, when the air pressure was only mode- 

 rately reduced, and the electrons would then be beyond the 

 control of the electric field. Thus the ionisation would not 

 be proportional to the air pressure, as was found by Durack 

 and McClelland. The difficulty as to the energy is not 

 obviated by supposing each primary ft particle to set free 

 only a few secondary electrons of high speed, each of these 

 to become in turn the originator of a few more, and so on. 

 For if that were the case, a reduction of gas pressure would 

 imply, not only that each primary electron set free fewer 



