DISCOVERY AND NATURE OF RADIOACTIVITY" 7 



Positive Ions from Radium ; Alpha Rays : It was Strult '^^ 

 who first suggested that the less deviable, or a rays were streams of 

 positively charged particles, and the experimental confirmation of 

 this hypothesis was reported by Crookes^^ in the following year. 

 One year later Rutherford ""^^ demonstrated that the a. rays could be 

 deviated in a magnetic field, and in a direction opposite to that of the 

 ^ rays, thus further confirming the fact that they carry a positive 

 charge.* The a. particles from radium were shown to have twice 

 the mass of a hydrogen atom. Further communications by Thom- 

 son"^'"^ and by Rutherford"^ established beyond reasonable doubt that 

 the a. rays of radium and other radioactive substances, consist of a 

 stream of positively charged particles, shot off from radioactive 

 bodies, but with a velocity much less than that of the electrons. 

 Rutherford ^^'^ has recently calculated that the velocities of expulsion 

 of the a particles from various radioactive substances all lie within a 

 range of 1.56 X lo** and 2.25 x 10^ cm. per second.! Thomson '^^ 

 also showed that bodies struck by the a rays from polonium have a 

 positive charge communicated to them. Because of their greater 

 size and less velocity, the particles of the a. rays are much less pene- 

 trating than are those of the /9 rays. Rutherford and Grier ^^^ have 

 shown that a layer of aluminium .09 mm. thick completely absorbs a 

 rays. Not all of the a particles have the same velocity, for, since 

 their motion is retarded by passing through matter, those coming from 

 different depths of a thick layer of radium will have unequal veloci- 

 ties, those from the surface moving fastest. For this reason a. rays 

 like /9 rays are not simple but complex.:]: The a. particle, losing its 

 positive charge, becomes a helium atom.'"^" 



Thomson found, in the course of his experiments, that both 

 polonium and radium emit slowly moving cathode rays which cannot 

 penetrate aluminium foil easily penetrated by /? rays. Swinton ^^* 

 has recently shown that anode rays that have passed through perfora- 

 tions in the cathode terminal (" canal rays ") may cause rapid motion 

 of mill wheels, just as Crookes demonstrated for cathode rays. 



* Experiments indicate that the a particle acquires its positive charge as the result 

 of ionization through collision. See Bibliography, p. 14, Nos. 17, 18, 113, and 77. 



Whether the a particle is a molecule of hydrogen, an atom of helium, or a helium 

 molecule carrying twice the ionic charge, is not easily decided (Rutherford"*), but 

 the results obtained by Cameron and Ramsay'* seem to indicate that the a particle and 

 helium are not identical. 



tBy the photographic method Des Coudres^' found the velocity (V) of the a 

 particles from radium to be 1.65 X lo' cms. per second. 



t Cf. citations Nos. »i7, 12, 21, ii6. 



