124 THE LIMITATIONS OF SCIENCE 



atom, which would serve satisfactorily as a source and, 

 at the same time, as a receptacle of radiant energy and 

 an ether which would transfer it. Not one of these 

 models has been even partially adequate; the course of 

 the development has been steadily from the simple to 

 the complex, from the concrete to the abstract, from 

 the physical to the metaphysical, until the most recent 

 atom is a complex more intricate than a stellar cos- 

 mogony, whose parts are an entity called electricity, and 

 the ether is an abstraction devoid of any mechanical 

 attributes. Out of all this controversy we have gained 

 the following facts : Heat, light, and electrical energy, 

 originating in one body, may be assumed to pass 

 through space undiminished and unaugmented to an- 

 other body. We can also express this energy as kinetic 

 energy while it is associated with matter. In transit, 

 since our experience gives us no clew or criterion, we 

 can assume as a formula for the energy, either a 

 periodic motion of an hypothetical something, called 

 an ether, or a projectile motion of an hypothetical mass- 

 particle. In either case, all we really do is to divide 

 the initial or final material energy into two mathe- 

 matical quantities, one a mass-factor and the other a 

 velocity-factor, and give to each such a value as to 

 make their product remain a constant. As a rule, we 

 make the mass-factor so small that we can shut our 

 eyes to its existence and imagine anything about it 

 we please. The time relation is fixed by experiment. 



