401 //. A. Bumstead — Emission of Electrons by 



more rapidly than where the ionization was less. As the knee 

 is more conspicuous in hydrogen than in air, the relative stop- 

 ping effects of these two gases are easily accounted for on this 

 hypothesis. 



Shortly afterward Geiger* published the results of a repeti- 

 tion of Kutherford's measurements of the velocities of the a- 

 particle at different points in its range. The measurements 

 were made under more favorable conditions than were possi- 

 ble at the time of Rutherford's original experiments, and 

 Geiger was able to show that the loss of energy by the particles 

 in passing through a layer of air was at least approximately 

 proportional to the ionization produced in that layer, and was 

 not the same for equal paths in air at different parts of the 

 range as Rutherford had supposed. The large value of the 

 ionization as the a-particle approaches the end of its range, 

 involves a correspondingly rapid diminution of its energy, and 

 this rapid expenditure of energy in all probability brings the 

 a-particle to rest at the end of its range instead of leaving it 

 with a residual velocity after it ceases to ionize, as Rutherford 

 at first supposed. 



The continuity of the effects observed by Taylor with 

 hydrogen, air, and various metals of increasing atomic weight 

 made it probable that, in solids as well as gases, the a-parti- 

 cles expend their energy in a process analogous to gaseous 

 ionization, and that this process follows a curve similar to the 

 ionization curve in gases. Just as the curve for air has a less 

 conspicuous knee than the hydrogen curve, so, from the stop- 

 ping effects, we should expect the " ionization curve " for gold, 

 for example, to show less projection than the air curve. 

 According to this view all atoms are less effective in retarding 

 swift rays than slow ones ; but heavy atoms are relatively 

 more effective in stopping the swift rays than the light atoms, 

 while they are on more nearly equal terms in their effect upon 

 the slower rays. This view accounts satisfactorily for the 

 results obtained by Taylor. 



The following experiments, were undertaken to obtain, if 

 possible, some direct evidence of the existence of an ionization 

 process in metals subjected to a-rays, and to see whether it 

 varied with the speed of the rays in a manner analogous to the 

 ionization which the rays produce in gases. It has been known 

 for some years that a source of a-rays, or any solid struck by 

 them, emits slowly-moving electrons (called S-rays by Sir J. J. 

 Thomson, who discovered them). 



If the atoms of solids are ionized by the rays one would 

 expect that some of the electrons set free in the process from 

 the atoms near the surface would escape from the metal, and 

 *Proc. Eoy. Soc, lxxxiii, 505 (1910). 



