SYMPOSIUM OX R.\DIOACTIVITY. 51 



temperature in radium compounds is due practically entirely to 



the internal bombardment of the mass by the a-particles, 

 southerns' experiment. 



Thomson has shown that the apparent mass of a moving elec- 

 tron is 



- _\ o yi ^ ^ ' V , 4 1 e- 



m -f 2/3 -T~ = m ' + .^ , ^ ., m air. 



where 

 e = change in e. s. units. 

 ^ =permeabilit}- in e. m. units, 

 a = radius of electron. 

 V = speed of hght. 



Hence, if the electron moves with the speed of light its kinetic 

 energ)-, in so far as it is due to electro magjietic mass, is just equal 

 to its electrostatic energ>\ And in general he showed that the 

 electro magnetic mass of any system is proportional to its poten- 

 tial energ}-. 



But Rutherford's measures have shown that in the process of 

 disintegration a mass of radium gives out an amoimt of energ>' 

 equal to 1/13,000. of the energ}- which this mass zi-ould have if 

 it were moving with the speed of light. Hence one might expect 

 its mass to be increased by 1/13,000. over that of an equal weight 

 of non-radio active substance. 



These considerations led Southerns to swing a pendulum under 

 two conditions as nearly identical as possible in all respects save 

 one : in the first case the pendulum bob was filled w^ith some 

 two pounds of uranium oxide (which is about 85 per cent ura- 

 nium, and supposedly about SO per cent radium"), and in the 

 other case the pendulum bob was filled with lead oxide. South- 

 erns'' results show that if the ratio of mass to weight for ura- 

 nium oxide differs from that for lead oxide, then the difference 

 is less than one part in '300,000. — and ven*- much less than that 

 predicted by the theory outlined above. 



COXCLUSIOX. 



In concluding let me say that nothing could be wider of the 

 mark, nothing more false, and therefore nothing more unfortu- 

 nate, than to imagine that since our ideas of the conservation of 

 energv' have been widened, since our notions of atomic individ- 

 uality have been enlarged, since our belief regarding the inde- 

 pendence of the elements has changed, and since our conceptions 



^Southerns Proc. Roy. Soc. A. M, 325 (1910). 



