THE SCIENTIFIC METHOD 135 



I shall, therefore, assume that the electron has a 

 constant mass, m f and that it possesses a force of gravi- 

 tational attraction for all other electrons which is 

 determined only by their masses and the distance be- 

 tween them. 



The electron, in addition to gravitational attraction, 

 has a power of electrical attraction. To measure this 

 force, I shall assume that it possesses a quantity of 

 electricity, e. Electrical force obeys the same law as 

 gravitational force, since it is conditioned by the quan- 

 tity of electricity and the distance between electrons. 



The electrical property of matter is also manifested 

 to us by the experimental fact, that an electrified body 

 apparently possesses a greater mass when in motion 

 than an unelectrified body. A similar effect is noticed 

 when a body is moved in a fluid. As the apparent in- 

 crease in mass is there due to the resistance of the 

 medium, it is called hydrodynamic mass, we may call 

 the apparent increase in mass of a moving electron, its 

 electro-magnetic mass m e . The total or effective mass 

 is therefore m'-f m e . 



Since the total mass is found to vary with the 

 velocity of an electron, two suppositions are possible. 

 Most physicists now assume that the mass, m, is a 

 variable and that the mass, m e) is constant. It seems 

 to me more rational and more convenient to adopt the 

 converse idea that the electro-magnetic mass, like the 

 hydrodynamic mass, is a variable. I shall, therefore, 



