328 COMSTOCK— THE MODERN THEORY [April 22, 



chanics, on the other hand, the mass of a body is considered to be a 

 constant and to have nothing whatever to do with the velocity. 



By studying experimentally the deflection of the beta-rays of 

 radium, which consist of streams of electrons travelling at veloci- 

 ties very near that of light, Kaufmann has shown that the experi- 

 mental change in mass fits the mathematically deduced change 

 wdien, and only when, the '* ordinary mechanical mass " is negligible. 

 In other words, as near as measurement can yet go, the mass of the 

 electron is enfirclx the electromagnetic mass of the surrounding 

 energy, and it has no appreciable mass of what I might call " the 

 old-fashioned mechanical kind." 



This is a result of extraordinary importance in physical theory, 

 for it immediately suggests the general cjuestion, " Is all mass of this 

 origin ? " Since an ordinary piece of matter is permeated with 

 electrons and also with the radiant energy which all parts of it are 

 constantly absorbing and emitting, it is an absolutely unavoidable 

 conclusion that at least part of the total mass is of this electro- 

 magnetic type. I')Ut the question is, " Is all mass electromagnetic?" 

 or, more properly speaking, " Is all mass of the saiiic type, and docs 

 it all depend upon the velocity in this same -iCay;^ " An aflirma- 

 tive answer would imply a profounder unity in physical phenomena 

 than has hitherto been recognized and would thus correspond to the 

 passage to a deeper synthesis. Of course, the deeper concept which 

 unites two or more others should, in strictness, be made independent 

 of these others ; but, although definitely foreshadowed in the present 

 case, the detailed statement of this deeper law is at present im- 

 possible except as regards chaiu/cs due to motion; so that this, 

 taken with the fact that electromagnetic phenomena are so familiar 

 that we may be said to know their }nodus operandi in terms of 

 magnetic fields, electric forces, and the like, renders it provisionally 

 allowable to state the question in the form: "Are the laws of elec- 

 tricity and optics the laws of matter in general ? " 



We have, during the last few years, been attaining with greater 

 and greater surety a definite answer to this question. It has come 

 through the gradual adoption of a remarkable concept, profound in 

 its meaning and very far-reaching in its consequences. I refer to 

 the so-called " l^-inci])le of Relativity." 



