328 Bumstead and McGougct/n — Emission of Electrons, etc. 



lar process takes place when gases are ionized by a-rays. The 

 column of ions following the track of the a-particle, which was 

 assumed by Moulin, and which has recently been made visible 

 in the beautiful experiments of C. T. R. Wilson, would then 

 be made up of the tracks of many secondary electrons, radiat- 

 ing irregularly from the axis of the column, and extending only 

 a small fraction of a millimeter from it. 



Summary. 



1. The number of 8-electrons emitted by a metal when struck 

 by a-ra}'s varies with the speed of the a-rays in the same man- 

 ner as does the number of ions produced in a gas. Curves which 

 represent this variation are similar in form to the Bragg ion- 

 ization curve. 



2. Such curves have been determined for aluminium, copper, 

 gold, lead and platinum. Within the limits of accuracy of the 

 experiments the} 7 have the same form for all these metals ; this 

 agreement is in contrast with the fact that ionization curves for 

 different gases and vapors show marked differences in form. 



3. The magnitude of the 8-ray effect, and the field necessary 

 to cause saturation, decrease progressively for some time after 

 the vacuum is produced. This is ascribed to the gradual removal 

 of a layer of adsorbed gas ; during this process the form of 

 the "ionization curve" does not change. This suggests that the 

 whole effect may be due such a layer which can not be entirely 

 removed ; the similarity of the curves obtained with different 

 metals would thus find a natural explanation. However, an 

 experiment, in which a strip of platinum was heated to red heat 

 in a high vacuum, failed to give evidence of the removal of this 

 assumed film. 



4. Polonium and the active deposit of thorium emit a radiation 

 which is completely absorbed by 064 X 10" 4 cm of aluminium. 

 This radiation causes the emission of electrons from metals 

 which it strikes. It consists of two portions, one of which is 

 primary and not appreciably affected by an electric field ; the 

 other is secondary and consists of electrons whose velocities range 

 from less than 3*8 X 10 8 to more than 24-4 X 10 8 cm/sec. 

 Wertenstein has found that Ra C emits similar soft radiations, 

 which ionize a gas ; he attributes the primary radiation to the 

 recoil atoms. 



5. The secondary radiation is emitted not only by the source, 

 but by any object on which a-rays are incident. The elec- 

 trons of which it consists have considerably higher velocities 

 than the 8-rays hitherto recognized. 



6. A part, at least, of the ordinary 8-radiation is due, not to 

 the direct action of the a-rays, but to these secondary rays. So 

 far as can be concluded from the present experiments, it is quite 

 possible that the whole of the 8-ray effect may be thus produced. 



