the Radioactivity of Radium. 211 



those of the second class attribute a considerable portion of 

 it to kinetic energy existing as such in the atom. The object 

 of this note is to emphasize the fact that these two classes lead 

 to essentially different conclusions as to the proportion in 

 which the energy emitted by radium is distributed between 

 a and (3 particles ; and so can be subjected to the test of 

 experiment. 



In the March number of this journal* Lord Kelvin points 

 out that on his latest theory the bulk of the heating effect of 

 radium must be attributed to the "ft particles.'''' This con- 

 clusion obviously rests on the assumptions that before the 

 explosion the "ft particle" and the positive residue of the 

 atom are at rest, and that during the explosion the law of 

 conservation of momentum holds between them. "With these 

 assumptions it may at once be extended to all theories of the 

 first class, according to which the positive part of the atom 

 has little kinetic energy initially and acquires relatively little 

 during the explosion. 



During explosion, however, the accelerations are very large; 

 and on an electromagnetic theory such as that of Lorentz, 

 imply large radiation of energy and of momentum into the 

 aether; so that the law of conservation of momentum onl}- 

 holds strictly when the aether is taken into account. But an 

 estimate of the error due to this cause has led me to the con- 

 clusion that it can hardly amount to more than one thousandth 

 of the whole, so that Lord Kelvin's conclusion is not thereby 

 affected to any material extent. 



On the other hand, a theory of the second class which 

 assumes that the greater portion of the kinetic energy of the 

 a particles exists as kinetic energy in the atom, at once leads 

 to the result that the bulk of the energy emitted by radium 

 is due to the a particles. It obviously requires that the atom 

 contain positive charges moving with orbital velocities com- 

 parable with the velocities of the a particles, or negative 

 charges moving with a velocity exceedingly nearly equal to 

 the velocity of light. The latter alternative in addition 

 requires some mechanism by which at explosion the kinetic 

 energy of the negative charges in the atom is transferred to 

 the a. particles. 



Rutherford t states that the heating effect and kinetic 

 energy of the /3 particles is probably not more than 1 per cent, 

 of that of; the a. particles, basing his conclusion partly on 

 theoretical considerations, partly on the measurements of 



* Phil. Mag. [6] vol. xiii. p. 314. 



f ' Radioactivity,' §§ 144, 247. (The references are to the German 

 edition, as I have not access at present to the 2nd English edition on 

 which it is based.) 



