352 GREAT MEN OF SCIENCE 



which an electrically heated platinum wire had been mea- 

 sured as regards its radiation from temperatures below red- 

 ness up to the melting-point of the platinum. Stefan found 

 from all such measurements without exception, that the 

 total radiation from a black body is proportional to the 

 fourth power of its absolute temperature (that is, measured 

 from-273°C); this rule has since been known as 'Stefan's 

 law.' Stefan was also able to state, on the basis of some good 

 measurements, the number of calories which one square 

 centimetre of surface of a black body radiates in one second, 

 and since the radiation arriving on the earth from the sun 

 had likewise been measured in calories, he was able by means 

 of his law to make for the first time a well-founded statement 

 concerning the temperature of the sun. He found it to be 

 approximately 6000° C (absolute), the sun being assumed to 

 have the properties above defined of a black body, which 

 agrees with all recent experience. 



Boltzmann succeeded in making use of an imaginary ex- 

 periment to bring Stefan's law, which had been derived 

 from experience, into quantitative connection with Clausius' 

 second law of thermo-dynamics, and with the pressure of 

 radiation derived from Maxwell's theory (1884). This was 

 of great importance, since Stefan's law suddenly ceased to 

 depend for support only on the special observations which 

 he used, and took its place in the firmly constructed theo- 

 retical structure formed by the theories we have mentioned. 

 This had the greater significance inasmuch as not long after, 

 Hertz was able to bring a further experimental confirmation 

 of Maxwell's theory. 



In this way, twenty-four years after Kirchhoff's law of 

 emission and absorption had been firmly founded, Stefan's 

 law could be added as a second and equally well-established 

 law concerning the ether wave radiation of hot bodies. From 

 this point progress was now much more rapid, the more 

 so as Boltzmann had already left behind two essential 



