to POPULAR SCIENCE MONTHLY. 



The evidence that the a particles are atomic in size mainly rests 

 on the deflection of the path of the rays in a strong magnetic and 

 slectric field. It has, however, been suggested by H. A. Wilson that 

 the a particle may in reality be a ' positive ' electron, whose magni- 

 tude is minute compared with that of the negative. The electric mass 

 of an electron for slow speeds is equal to 2e 2 /3a. Since there is 

 every reason to believe that the charge carried by the a particle and 

 the electron are the same, in order that the mass of the positive elec- 

 tron should be about 2,000 times that of the negative, it would be 

 necessary to suppose that the radius of the sphere over which the 

 charge is distributed is only 1/2000 of that of the electron, i. e., 

 about 10 — 10 cms. The magnetic and electric deflection would be 

 equally well explained on this view. This hypothesis, while interest- 

 ing, is too far reaching in its consequences to be accepted before some 

 definite experimental evidence is forthcoming to support it. The evi- 

 dence at present obtained 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. 



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 rays emitted by them are deflectable. There is 

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

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

 rays from the different radioactive products differ in their power of 

 penetrating 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 e/m. Becquerel states that the 

 amount of curvature of the a rays from polonium in a field of con- 

 stant 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 



* Bragg and Klceman (Phil. Mag., Dec, 1904) have recently attacked 

 this question and have offered a very satisfactory explanation of the mechanism 

 of the absorption of the a rays by matter. 



