376 GREAT MEN OF SCIENCE 



mass like that of the electro-magnetic energy of light waves, 

 the total mass of a moving body must increase when its 

 velocity is increased, since there is added to the mass which 

 it possesses in a state of rest the ever increasing mass of the 

 kinetic energy which it contains. If this increase of mass is 

 calculated, by means of Hasenohrl's result, with the addition 

 of Newton's second law of motion,^ we find that the increase 

 of mass only becomes perceptible at very high velocities, 

 approaching that of light; at the velocity of light it would 

 actually become infinite. In order to be able to test whether 

 the calculated increase is actually present in reality, it was 

 necessary to find masses which could be given very great 

 velocities, which velocities could also be increased at will. 



Such masses exist in the cathode rays, which are quickly 

 moving electrons. In the course of measurements of great 

 difficulty and increasing refinement, velocities were investi- 

 gated which reached to within a few per cent, of that of 

 light, and the increase of mass was found to be exactly that 

 calculated from the kinetic energy. ^ This is also the par- 

 ticular reason derived from experience, for assuming that the 

 calculation of the mass of energy is true for all forms of the 

 same. This shows that the velocity of light cannot be 

 exceeded by an electron, and therefore no doubt also not by 

 matter, which is built up of elementary electric charges; for 

 in order to attain the full velocity of light, an infinitely great 

 expenditure of energy would be necessary. 



The proportionality between mass and weight already 



1 I earned out the very simple calculation (Ather und TJrdther, 

 Leipzig, 1922, pp. 48, 49) in order to destroy if possible the impression 

 which is widely prevalent that a peculiar, very complicated, and not 

 easily understood theory, originated by Hasenohrl, is necessary to it. 

 I later found that Hasenohrl himself already gave this simple calculation 

 (Stark's Jahrbuch, vol. 6, 1909, p. 501), in which he refers to another 

 author, who carried it out in 1908 after his fundamental publication. It 

 is still more remarkable that even to-day, when these results are used, no 

 mention is made of Hasenohrl. 



2 Here we must replace Hasenohrl's factor 4/3 by i, as we can also see 

 iiom the simplified calculation from the pressure of light. 



