On Mass, Energy, and Radiation. 679 



suggested, but the balance of internal evidence in the paper 

 points to faulty pumping as the cause. In filling lamps on a 

 Toepler pump, I always leave the lamp evacuated at the 

 pump in connexion with a large P 2 5 tube for at least 

 24 hours before running in the metal in order to be sure of 

 the removal of water vapour. I also usually cast the cadmium 

 into sticks by suction into a heated glass tube fitted with a 

 tap in order to be sure of the absence of blow-holes. The 

 process of filtration, if properly carried out, is quite sufficient 

 to free the molten metal from solid extraneous matter such 

 as oxide, and I should certainly have considered the operation 

 faultily performed if I had ever noticed traces of oxide in 

 the lamp such as Mr. Bates reports ]ie occasionally observes. 

 The peeling off of " thin sections of the quartz from the 

 walls/' reported by Mr. Bates, certainly points to the for- 

 mation of a cadmium-glass due to oxide as the result of water 

 vapour, and not to the action of adhering metal. 



I am, Gentlemen, 

 The Sir John Cass Technical Yours faithfully, 



Institute London H j g g 



March 20th, 1920. 



LXX. Mass, Energy, and Radiation. 

 By J. J. Thomson, O.M., P.R.S.* 



THE object of this pap^r is to endeavour to supply a 

 method of representing in terms of physical conceptions 

 the processes occurring in physical phenomena. It is an 

 attempt to help those who like to supplement a purely 

 analytical treatment of physical problems by one which 

 enables them to visualize physical processes as the working 

 of a model ; who like in short to reason by means of images 

 as well as by symbols. 



The ideas on which the method is based were suggested 

 by the consideration, from the electrical point of view, of 

 the origin of the mass of an electron. From this point of 

 view this mass is distributed throughout the region sur- 

 rounding the electron, and for an electron at rest the muss 

 per unit volume at any point in this region is proportional 

 to the square of the electric force at the point. The electro- 

 static potential energy per unit volume at this point is also 

 proportional to the square of the electric force and is thus 

 proportional to the mass. In fact (see 'Electricity and 

 Matter/ J. J. Thomson, Chap. 2) the electrostatic potential 



* Communicated bv the Author. 



