PRESENT PROBLEMS OF RADIOACTIVITY 163 



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 inclosed in a lead envelope 1 cm. thick,- a 

 thickness sufficient to absorb completely the ordinary /? rays emitted 

 by radium, but which allows about half of the f 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 

 the lead envelope. When the air was exhausted, this outer ring was 

 found to gain a negative charge. These experiments, at first sight, 

 indicate that the 7- rays carry with them a negative charge like the 

 /? rays. In order to account for the absence of deflection of the path 

 of the f rays in very strong magnetic or electric fields, it is necessary 

 to suppose that the particles have a very large apparent mass. Pas- 

 chen supposes that the -/- particles negative are electrons like the /? 

 particles, 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 Univers- 

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

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

 surface of which they emerge and also on the surface of 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 consisted, 

 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 receive 

 a simple explanation without the necessity of assuming that the f rays 

 of radium themselves carry a negative charge. The lead envelope 

 in his experiment acquired a positive charge in consequence of the 

 emission of a secondary radiation consisting of negatively charged 

 particles, projected with great velocity from the surface of the lead. 

 The electric charge acquired by the metal ring was due to the absorp- 

 tion of these secondary rays by it, and the diminution of this charge 

 in a magnetic field was due to the ease with which these secondary 

 rays are deflected. It is thus to be expected that the envelope sur- 

 rounding the radium, whether made of lead or other metal, would 

 always acquire a positive charge, provided the metal is not of suffi- 

 cient thickness to absorb all the /- rays in their passage through it. 



No conclusive evidence has yet been brought forward to show 

 that the ?- rays can be deflected either in a magnetic or electric field. 

 In this, as in other respects, the rays are very analogous to the Roent- 

 gen X rays. 



According to the theory of Stokes, J. J. Thomson, and Weichert, 



