JOSEF STEFAN AND LUDWIG BOLTZMANN 353 



preliminary investigations of wide importance, which con- 

 tinued to influence research until quite recently. By ad- 

 mirable and very extensive studies concerning the motion of 

 gas molecules, he was able, basing himself upon the previous 

 work of Clausius and Maxwell, to bring the concept of 

 entropy, so important in connection with the second law of 

 thermo-dynamics, into a definite relationship with the 

 value of the probability (as already defined by Huygens) of 

 the physical state of the group of bodies considered, whereby 

 a new foundation was given for the application of the con- 

 cept of entropy to further deductions. Furthermore, Boltz- 

 mann in the course of these investigations, and in the same 

 year (1877), had also introduced a peculiar mode of calcula- 

 tion for discontinuous or quantum-like distribution of 

 energy among gas molecules, which later became of especial 

 importance when somewhat differently applied.^ 



The most important questions concerning the radiation 

 of hot bodies remaining open after Boltzmann's time related 

 to the distribution of the total radiation given by Stefan's 

 law among the various wave-lengths in the spectrum. The 

 simplest observation of a body gradually heated to incan- 

 descence, such as a piece of carbon or iron, already shows 

 us that from the beginning of a red heat (about 6oo°C) 

 we have the addition of red radiation to the hitherto in- 

 visible infra-red, then yellow appears, until finally at a white 

 heat, the blue and violet also appear in the spectrum. The 

 range of rays sent out thus gradually moves in the direction 

 of shorter and shorter v/ave-lengths as the temperature rises. 

 The law of this displacement could again be deduced by 

 means of an imaginary experiment from already well- 

 founded knowledge; it was now necessary to make use, in 

 addition to Clausius' second law of thermo-dynamics, and 

 Maxwell's pressure of light, also of Doppler's principle. The 



^See Boltzmann's scientific papers, edited by F. Hasenohrl, vol. 2, 

 page 167. 



Zs 



