378 GREAT MEN OF SCIENCE 



mass of all substances, of all atoms including the non- 

 radio-active, is only energy mass, so that also all ordinary 

 weight is only energy weight. For our experience with 

 cathode rays has shown strong electro-magnetic fields of 

 force to exist in the interior of all atoms, and even the centres 

 of these fields of force, the negative and positive elementary 

 charges, are themselves to be regarded as concentrations of 

 energy. It was further hardly to be doubted that the almost 

 exact whole numbers of the atomic weights is the result of 

 the atoms containing whole numbers of elementary quantum 

 pairs,^ while the - generally small - departures from whole 

 numbers depend upon differences in the content of energy, 

 and hence the weight, of the electro-magnetic fields of the 

 pairs. 2 



If we accordingly regard ether and matter from this point 

 of view, the distinction between the two, which together 

 make up the material world, may appear to be obscured; for 

 in both of them energy and hence mass and weight have 

 been found, although in very different degrees. We have 

 taken away from matter its hitherto imagined distinction 

 from ether. Matter appears according to the facts we have 

 just summarised only as a special case of energy, and we now 

 have to set side by side not matter and ether, but rather 

 energy and ether. 



Of matter we must say that its atoms represent enormous 

 concentrations of energy, which however for the most part 

 must be looked upon as untransformable. Only the 

 heaviest and radio-active atoms emit energy of themselves, 



^ Such a pair, consisting of a positive and negative elementary charge 

 with the corresponding field of force, regarded as a fundamental con- 

 stituent of all atoms, has also been called a 'dynamid.' Hence we may, 

 at the present time, regard it as established that gravitation and inertia 

 are properties belonging to energy and to it alone. These properties 

 were at first discovered only in the case of matter, because its atoms are 

 enormous accumulations of energy. 



2 A full account of these matters will be found in my paper Vber 

 Energie utid Gravitation, Heidelberg Academy 1929. 



