[m'lennan] on electric CHARGES 83 



In view of the identity established by Eutherford between the alpha 

 particles and the atoms of helium, it would follow from the view taken 

 by Strutt — ^that the origin of helium in the saline minerals is the po- 

 tassium which they contain — that rays of the alpha rays should be pre- 

 sent in the radiation from potassium salts. 



With the exception of Henriot no one, however, seems to have ob- 

 served any indication of the presence of such rays. Usually, however, 

 these rays have been detected and identified either by their fluorescent and 

 ionising action or by the deflections which they undergo in electric or 

 magnetic fields. But for these methods to be applicable the alpha par- 

 ticles must have velocities which will take them away a few millimetres 

 at least from the substances which emit them. 



It is quite possible then that potassium and even other substances 

 may be emitting alpha particles with exceedingly low velocities, and that 

 these rays have hitherto escaped detection largely through the inade- 

 quacy of the means adopted to bring them into evidence. 



In looking for a means of demonstrating the poss.ible existence of 

 these rays it seemed to the writer worth while to make an attempt to in- 

 vestigate the radiations emitted by the potassium salts, by examining 

 them for the acquisition of an electric charge when placed on insulating 

 supports within a highly exhausted vessel. 



Amongst others M. and Mme Curie,i Paschen,^ Strutt,^ Aschkinass * 

 and Makower ^ have applied this method with success to the investiga- 

 tion of different tj'pes of radiation. In Paschen's experiments it was 

 found when a lead cylinder containing a small quantity of a radium salt 

 was insulated in a vessel from which the air was removed that the lead 

 cylinder acquired a positive charge through the action of the rays which 

 were emitted. This effect was thought by Paschen to prove that the 

 gamma rays from radium consisted of streams of rapidly moving nega- 

 tively charged particles, but this was afterwards shown by Eve ^ to be due 

 to the excitation and consequent emission of a secondary radiation of the 

 beta type in the lead by the passage through it of the gamma rays. 



The method, too, was applied by Strutt (loc. cit.) in his interesting 

 experiment popularly known as the radium clock, and more recently it 

 was applied by Aschkinass to demonstrate the existence of the delta rays 



1 M. & Mme. Curie Comp. Rend. CXXX, p. 647, 1900. 



2 Paschen, Wied. Ann. 14, 1, pp. 164-171, 1904. 

 s Strutt, Phil. Mag., Nov. 190.3. 



4 Aschkinass, Phys. Zeit. 8, p. 773, Oct. 24, 1907. 

 B Makower, Phil. Mac., Jan., 1909. 

 6 Eve, Nature, Sept. 8, 1904. 



