BY J. L. GLASSON, B.A., D.SC. S 



1913. 



assuinption (Proc. Roy. Sor.^ vol. 83, p. 513). It appears, 

 then, that Whiddington's results (ii) regarding the loss of 

 energy in traversing matter might have been deduced from 

 the law (i) connecting ionisation and velocity, and vice versa. 



AIJ the constants in (v) are known, so that we can 

 calculate Q. 



For air at 760 m.m. pressure Whiddington gives a 

 = 2-0 X 10*0; andw = 8-8 x 10-28 ums., /.• = 2*5 x 10<2. 

 therefore Q = VIQ X IQ-io. 



Expressing this in the usual way as a fall of potential in 

 volts by the relation eV == ^ mv^ = Q we get F = I'l x 

 10^ =110 volts. So that Jot each pair of ions made the 

 ray experiences a loss of velocity corresponding to a fall 

 threucjli 110 VOLTS. 



We may compare these results with those obtained for the 

 a rays. Geiger has shown {Proc. Roy. :Soc., vol. 84, p. 505) 

 that for the rays from Ra. C. the ionisation per cm. of path 

 varies inversely as the velocity of the rays. From his data 

 I have calculated that the energy lost by the ray per ion 

 made is O'o x lO-^i ergs. For ^ rays Q = 1-76 X lO-io. 

 So that in making a single ion a J3 ray will lose three times 

 as much energy as an a ray. 



There have been many estimates of the energy required 

 to produce an ion, varying from 5 volts up to several hundred 

 volts. Perhaps the most probable value is that given by 

 Townsend, viz., 10 volts. If this value be accepted we see 

 that the proportion of the energy lost by the ray which is 

 actually spent in ionising is fairly small. It seems probable 

 that the bulk of the energy lost by the ray is spent in setting 

 the electrons within the atoms of the gas into vibration 

 insufficient in amplitude to cause their ejection from the 

 atom. This energy of course appears ultimately as heat. 



This paragraph is devoted to a consideration of the pro- 

 portion of the energy of a cathode ray which is spent im 

 ioBieation. 



Sir J. Thomson has shown {Phil. J\Ja(j., April, 1912),, 

 that the number of ions made by a cathode ray in traversing 

 uiait length of air is given by the expression 



nire^ , ,v 



wkere n = number of corpuscles in I c.c. of air, 

 W = energy required to ionise an atom, 

 T = kinetic energy of the moving ray (= h nn*).. 



