162 PHYSICS OF THE ELECTRON . 



in its consequences to accept before some definite experimental 

 evidence is forthcoming to support it. The evidence at present ob- 

 tained strongly supports the view that the a particles are in reality 

 projected matter, atomic in size. The probability that the a particle 

 is an atom of helium is discussed later, in section vm. 



Becquerel showed that the a rays of polonium were deflected by a 

 magnetic field to about the same extent as the a rays of radium. On 

 account of the feeble activity of thorium and uranium, compared 

 with radium and polonium, it has not been found possible to examine 

 whether the a rays emitted by them are deflectable. There is little 

 doubt, however, that the a particles of all the radio-elements are pro- 

 jected matter of the same kind (probably helium atoms). The a rays 

 from the different radioactive products differ in their power of 

 penetration of matter in the proportion of about three to one, being 

 greatest for the a rays from the imparted or "induced" activity of 

 radium and thorium, and least for uranium. This difference is prob- 

 ably mainly due to a variation of the velocity of projection of the 

 a particles in the various cases. The interpretation of results is 

 rendered difficult by our ignorance of the mechanism of absorption 

 of the a rays by matter. Further experiment on this point is very 

 much required. 



It is of importance to settle whether the a particles of radium 

 and polonium have the same ratio of ~. Becquerel states that the 

 amount of curvature of the a rays from polonium in a field of constant 

 strength was the same as for the a rays from radium. This would 

 show that the product of the mass and velocity is the same for the 

 a particles from the two substances. The a rays of polonium, how- 

 ever, certainly have less penetrating power than those of radium, and 

 presumably a smaller velocity of projection. This result would indi- 

 cate that ^ is different for the a particles of polonium and radium. 

 It is of importance to determine accurately the ratio of ^ and the 

 velocity for the rays from these two substances in order to settle this 

 important point. 



IV. The f Rays 



In addition to the a and /? rays, uranium, thorium, and radium all 

 emit very penetrating rays known as f rays. These rays are about 

 100 times as penetrating as the /? rays, and their presence can be 

 detected after passing through several centimeters of lead. Villard, 

 who originally discovered these rays in radium, stated that they were 

 not deflected in a magnetic field, and this result has been confirmed 

 by other observers. Quite recently, Paschen has described some 

 experiments which led him to believe that the f rays are corpuscular 



