96 



Lenard found the velocity of the 8 rays due to ultra-violet 

 light to depend somewhat on the nature of the light ; but it is 

 hard to believe that a diminution of the width of the pulse, 

 no matter how extreme, can increase the energy of the ejected 

 electron about a thousand times. 



But if we regard the secondary radiation as the result of 

 the break-up of a neutral pair, the high velocity of £he ejected 

 electron (5 x 10^) may be more readily explained. The action 

 must be entirely different from that of ultra-violet light. 



It is difficult to found any arguments for or against either 

 theory on considerations of the relative energies of the origi- 

 nal cathode stream, the X-rays, and the secondary rays. For 

 if the energies of any transformation do not balance, it is 

 easy to square the account by postulating either some release 

 of the internal energy of the atom, or the reverse, viz., the 

 absorption of energy by the atom involving an apparent dis- 

 appearance of the visible energy. In the case of the ether- 

 pulse theory it is necessary to suppose that the secondary 

 radiation derives its energy from the atom's internal store 

 (Conduction of Electricity through Gases, 2nd Ed., p. 321). 

 Also, as mentioned in the previous paper, there are difficulties 

 in reconciling the proportion of X-ray energy to cathode-ray 

 energy, with the probable circumstances of the impact of the 

 cathode rays. (Wien ; Ann. d. Phys., xviii., p. 991, 1905, 

 and xxii., p. 793, 1907; van der Waals, Jr.; xxii., p. 603, 

 1907). On the neutral-pair hypothesis, the cathode rays 

 would probably cause the release of the pairs, which would 

 derive much of their energy of motion from the atom ; it 

 might not be necessary to invoke the aid of the atomic energy 

 in order to account for the energy of the secondary radiation. 



There is another entirely different argument, which seems 

 to support the neutral-pair hypothesis. 



The a, ^, and y rays all ionize the gases which they tra- 

 verse. It has just been shown by Kleeman (D that the 

 ionisation per atom due to fi and y rays is nearly 

 proportional to the ionisation per atom due to 

 a rays (and, therefore, anproximately proportional to 

 the volume as I have shown, Proc. Roy. Soc. of S.A., Oct., 

 1906; Phil. Mag, March, 1907). The figures for the heavier 

 atoms are rather larger for the /? than the a rays, and 

 still larger for the y rays. It is known that the ionisations 

 due to X-rays differ considerably from those due to y rays 



(1) Mr. Kl<>eman has been good enough to inform me of his 

 results by l^-tter ; but I believe I -am at liberty to quote them, since 

 he has, 1 understand, recently read a paper on the subject beforo 

 the Royal Society. 



