SUCCI.ssloX OF CII\V<;KX IN BAPIOACmVE BODIES. 



213 



pressure lielow a certain limit, and does not seem to te much altered if tlie air is 

 replaced by hydrogen. At the snme time, taking the estimate of the numl>er of 

 particles from 1 gramme of radium, disr.us.srd in section 31, it was deduced that the 



ElectromeCer 



To pump 



Fig. 16. 



charge carried by them should certainly have been detected by reversal of the electric 

 field, notwithstanding the presence of surface ionization. 



A large number of experiments have been made under different conditions, but the 

 results have so far all been negative. It is very difficult to avoid the conclusion that 

 the a particles do carry with them a positive charge, but it must be supposed that 

 the effect of this charge is in some way exactly neutralized. In this connection, it is 

 significant that the charge can-led by the ' canal rays,' which are analogous to the 

 a rays of the radio-elements, has not so far been detected, although careful experi- 

 ments upon that point have l>een made by WIEN. The apparent absence of charge 

 on the a particles would be explained if an equal number of negatively charged 

 particles, or electrons, were expelled at the same time with a slow velocity. If the 

 electrons had about the same penetrating power as the a particles, it would be 

 difficult to detect their presence by the electric method, as the ionization produced by 

 the a particles would probably mask that produced by the electrons. The electron 

 should be readily deflected in a magnetic field and experiments are, at present, in 

 progress to examine whether the a rays show any trace of positive charge when the 

 rays are exposed to a strong magnetic field. 



31. Magnitude nf the Changes Occurring in the ftadio-element*. I have shown 

 (' Radioactivity,' pp. 154-158) that probably about 10 11 of the a particles are expelled 



