FRIEDRICH HASENOHRL 377 



recognised by Newton at once suggests the question whether 

 the mass of energy possesses a corresponding weight in this 

 proportion, whether, therefore, energy is subject to gravita- 

 tion. This question has been answered in two ways in the 

 affirmative. In the first place, a deviation of the light from 

 fixed stars when passing close to the sun has been observed 

 at eclipses. The rays of light are bent towards the sun, and 

 as far as the difficult observations, which are influenced by 

 refraction in the sun's atmosphere, can show, about to the 

 extent that would be expected in the case of any body sub- 

 jected to gravitation and projected with the velocity of light 

 close past the sun. The energy mass of light thus appears 

 to exhibit, as far as we can see, a gravitation equal to that of 

 the mass of all heavy bodies. 



On the other hand, radio-active bodies, such as uranium, 

 were used as the bobs of pendulums, in order that in their 

 case the proportionality between mass and weight could be 

 tested as in the pendulum experiments of Galileo and New- 

 ton, and this proportionality was found to be confirmed. In 

 this case a distinctly sensible part of the mass of radio-active 

 atoms consists without doubt of energy; for these atoms con- 

 tinually emit great quantities of energy in the form of 

 the radiations and heat which they develop. If this part 

 of the mass of the atoms did not possess a corresponding 

 weight, the pendulum experiments would have shown a 

 discrepancy, which however was not the case. 



According to these results, which were obtained in con- 

 nection with Hasenohrl's investigations, energy behaves 

 exactly like a substance. It does not alter its quantity, as 

 Robert Mayer showed; it possesses inertia and also a cor- 

 responding weight. This immediately suggests that its 

 distribution in space should be investigated in each indivi- 

 dual case, and this can at once be carried out. We find that 

 energy in all its forms always exists only in electro-magnetic 

 fields of force. This allows us to assume that the whole 



