PRESENT PROBLEMS IN RADIOACTIVITY. n 



presumably a smaller velocity of projection. This result would 

 indicate that e/m is different for the a particles of polonium and 

 radium. It is of importance to determine accurately the ratio of e/m 

 and the velocity for the rays penetrating two substances in order to 

 settle this vital point. 



The y Rays. 



In addition to the a and ft rays, uranium, thorium and radium 

 all emit very penetrating rays, known as y rays. These rays are about 

 100 times as penetrating as the ft rays and their presence can be de- 

 tected 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 y rays are corpuscular 

 in character, consisting of negatively charged particles (electrons) 

 projected with a velocity very nearly equal to that of light. This 

 conclusion is based on the following evidence: Some pure radium 

 bromide was completely enclosed in a lead envelope 1 cm. thick — a 

 thickness sufficient to completely absorb the ordinary ft rays emitted 

 by radium, but which allows about half of the y rays to escape. The 

 lead envelope was insulated in an exhausted vessel and was found to 

 gain a positive charge. In another experiment, the rays escaping 

 from the lead envelope fell on an insulated metal ring, surrounding 

 it. When the air was exhausted, this outer ring was found to gain a 

 negative charge. These experiments, at first sight, indicate that the 

 y rays carry with them a negative charge like the ft rays. In order to 

 account for the absence of deflection of the path of the y rays in very 

 strong magnetic or electric fields, it is necessary to suppose that the 

 particles have a very large apparent mass. Paschen supposes that the 

 y rays are negative electrons like the ft rays, but are projected with a 

 velocity so nearly equal to that of light, that their apparent mass is 

 very great. 



Some experiments recently made by Mr. Eve, of McGill University, 

 are of great interest in this connection. He found by the electric 

 method that the y rays set up secondary rays, in all directions, at the 

 surface from which they emerge and also on the surface on which they 

 impinge. These rays are of much less penetrating power than the 

 primary rays and are readily deflected by a magnetic field. The 

 direction of deflection indicated that these secondary rays coi sisLed, 

 for the most part, of negatively charged particles (electrons) pro- 

 jected with sufficient velocity to penetrate through about 1 mm. of 

 lead. In the light of these results, the experiments of Paschen re- 

 ceive a simple explanation without the necessity of assuming that the 



