374 GREAT MEN OF SCIENCE 



wave-length, including those of light, is beyond all doubt; the 

 energy can always be measured in the form of heat by caus- 

 ing the radiation to be absorbed in a suitable body. It was 

 a matter of calculating the mass of this energy in the case of 

 these waves. 



Hasenohrl carried out this calculation by means of an 

 imaginary experiment with a hollow space, which was pic- 

 tured as filled with ether waves and given an acceleration. 

 Considerations, in which Maxwell's pressure of light played 

 an important part, this pressure being exerted by the radia- 

 tion enclosed in the hollow space upon the walls of the 

 same, allowed that part of the inertia due to the enclosed 

 radiation, that is to say, the mass of the energy of the radia- 

 tion, to be calculated. This mass is shown to be equal in 

 grams to the quantity of energy (measured in Gauss's 

 absolute unit) divided by the square of the velocity of 

 light.i This is Hasenohrl's important and highly remark- 

 able result. 



In this way the mass of electro-magnetic energy could for 

 the first time be calculated in a well-founded manner. But 

 the assumption could immediately be made that this calcu- 

 lation ought to hold for every form of energy; for otherwise 

 mass would disappear or appear out of nothing in the course 



* Hasenohrl's imaginary experiment and calculation of the same 

 {Berichte der Wiener Akademie, Vol. 113, 1904, and Annalen der Physik, 

 Vol. 15, 1904 and Vol. 16, 1905) is more ot less complicated. I have set 

 some value on the most completely attainable simplicity (Ather und 

 Urdther, Leipzig, 1922, pp. 41, 42), and I find that the same was done before 

 me but after Hasenohrl, on another side; but Maxwell's pressure of radia- 

 tion always remains the chief point. A simplification of the way by which 

 an already known result is reached is naturally a comparatively easy matter; 

 but it increases, and even opens up for the first time, our insight into 

 essential features. We find in Hasenohrl a factor 4/3 multiplying the mass 

 of energy, which factor we left out above, as being close to unity and in 

 accordance with the result of the simplified view; factors of this kind, 

 which on account of the novelty of the matter, cannot be calculated at 

 first with the completest possible approximation to reality, also appear 

 in allied cases, such as the kinetic theory of gases. Some uncertainty in 

 regard to them makes no difference to the main result, and they can then 

 be corrected later, often with the recognition of further new truth. 



