172 THE LIMITATIONS OF SCIENCE 



approaches that of light, then the effect of its velocity 

 on its shape, size, and mass is an important matter. 

 For, let m be the mass of an electron at rest, from 

 Professor Einstein's formulae, its apparent mass in the 

 direction of motion is 



m 



and its mass at right angles to the motion which would 

 make it resist change of direction is 



Thus we have the unusual result of not only a variable 

 mass, but also a difference in mass according to the 

 direction considered. 



Another consequence of the hypothesis is, that mass 

 also depends on energy. This result is of extraordi- 

 nary importance, for it means we have no ability to 

 distinguish between the inertia of a physical system 

 of bodies and its energy; in other words, between the 

 inertia of a body and the energy content of the space 

 surrounding it. When we recall Professor Einstein's 

 conception of space and energy, which I have quoted, 

 most persons will become convinced that no idea of 

 the nature of matter could be more abstract. The 

 mass of a single body thus remains constant only when 



