545 



introduce some changes, securing, at any rate, the independence of 

 my results. 



An investigation of crjstals seemed important to me, because 

 VON EöTVOs in his investigation of stibnite does not mention, whether 

 attention was paid to a definite orientation. The orientation relative 

 to the vertical however might be of supreme importance. An invest- 

 igation with radioactive substances is of interest, because it enables 

 us to verify the proposition that energy possesses mass. We know 

 that if A^" denotes a change of energy of a body, c the velocity 

 of light, then to t.E corresponds a change Lm of the mass given 



LE 

 by the formula Am = — . Because (?" is extremely great, we can 



by ordinary methods only obtain inappreciable changes of mass. 

 We cannot hope ever to be able to measure the changes of mass 

 caused by the effect of temperature or by chemical transformations. 

 In the case of radioactive bodies the processes of disintegration 

 entail losses of energy of another order of magnitude than in the 

 case of chemical transformations. During the transformation of uranium 

 into lead and helium an enormous amount of energy must be relea- 

 sed. This is already the case during part of the necessary transfor- 

 mations, for in the course of its life one gram of radium with its 

 transformation products including radium F emits about 3,7 10" 

 calories '). 



Ti- H . u r w 3,7X10^'X4,18X107 



I his corresponds to a change of mass equal to = 



F & H 9X10^" 



= 0,6 X 10-^ g per gram. 



If this energy possesses mass, but no weight, then pendulums 

 with lead, helium, uranium must give values for the acceleration, 

 differing by more than 1 part in 10,000. 



Already several years ago these considerations were given by 

 J. J. Thomson, who also made experiments with a pendulum the 

 bob of which was made of radium. It was, however, impossible to 

 obtain a high degree of accuracy as the quantity of radium available 

 was very small. Afterwards Southerns ') made experiments in 

 Thomson's laboratory with pendulums with uranium oxide and red 

 lead. He came to the conclusion that the ratio of mass to weight 

 for uranium oxide, does not diifer from that for lead oxide by more 

 than one part in 200,000. 



1) Rutherford. Radioactive Substances, p. 582. 



2) Southerns. Determination of the ratio of mass to weight for a radioactive 

 substance, Proc. Royal. Soc. London, A. 84, 325. 1910. 



