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proportional to its energy. I was able to show, several years ago, 

 that, with certain limitations, this same result is true for any electric 

 system. 



It may be said, then, to follow from what might be called ele- 

 mentary electromagnetic principles, that the electromagnetic mass of 

 an electric system is proportional to its electric energy. Now 

 Hasenhorl, at Plancks's suggestion, I believe, had already shown 

 that a similar result applied to radiant energy properly speaking; 

 for he found that from known laws of radiation it followed that a 

 hollow box, like that mentioned earlier, with perfectly reflecting 

 walls, would, when filled with radiant energy, act as if it had added 

 mass. That is, the pressure of radiation would be so changed by the 

 increasing velocity of the box as to oppose the force causing this 

 increase, and this inertia, this added mass, he found would be 

 proportional to the amount of energy present. My proportionality 

 constant agrees with his. Lewis, from a totally different point of 

 view, has reached a similar conclusion. 



It is, as you know, one of the profoundest generalizations in 

 modern physics that light and other forms of radiant energy are in 

 reality all forms of electromagnetic energy. Hasenhorl's results, 

 therefore, that confined radiant energy possesses mass, combined 

 with the result obtained in connection with the bound energy sur- 

 rounding electric charges, gives us the general result that all elec- 

 tromagnetic energy, whether bound or radiant, possesses mass, and 

 this mass is proportional to the quantity of energy present. 



You see that the concept of energy, although in some ways very 

 illusive, is getting singularly definite and persistent. Since we see 

 that electric mass is proportional to electric energy, the question 

 naturally arises : How much of the mass of the electron is due to 

 the electric energy surrounding its relatively enormous charge, and 

 how much is the "ordinary mechanical mass" of its body proper? 

 We have a means of distinguishing between the two masses, for 

 the electric mass does not remain constant when the velocity of the 

 charge becomes great. Electrical laws tell us that it increases, very 

 slowly at first, then more rapidly, and that as the velocity of light 

 is approached it becomes very great. Of course, in ordinary me- 



