908 Prof. EL A. B u instead on Emission of Electrons 



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 

 iis 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 

 <z-particles 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 stopping effects,, we should expect the kC ionization 

 curve [' of gold, for example, to show less projection than the 

 air-curve. According to this view all atoms are less effective 

 in retarding swdft a-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. ]t has been 

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

 struck by them, emits slow-moving electrons (called 8- rays 

 by Sir J. J. Thomson who discovered them). If the atoms 

 of solids are ionized by the a-rays one w r ould expect that 

 some of the electrons set free in the process from the atoms 

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

 number escaping would be more or less nearly proportional 

 to the number of atoms ionized. Accordingly, measurements 

 were made of the number of electrons escaping from a thin 

 metal foil when struck by a-fays at different points in the 

 range. 



In order to avoid possible complications from the effects 

 of ft- and 7-rays, polonium w 7 as used as the source of a-rays. 

 Professor Boltwood kindly separated the polonium from a 

 solution of radio-lead and it was deposited on the end of 

 a copper plug, 4 mm. in diameter. The first preparation was 

 not very strong and it was necessary to use a system of small 



