( 668 ) 



-'^./{XT),!). (1) 



w hero /'(/. 7'j is a riiiictioii of tlit' jn-odiict /. 7'. Al'lcrwards Planck ') 

 li.is IoiiihI tor (1) tlu' form 



8 ^ f A 1 



iloi'O c is the \elocilv ot' light in aetlier and // i[.\\i\ k are univer- 

 sal constants. 



In tiic theory {^ï Planck every ponderable body is supposed to 

 coutain a great many electromagnetic vil)rators, or, as Planck calls 

 ihcm, "resonators", each of which has ils own ])eriod of free vibra- 

 tion, and which exchange energy willi Ihe aether as well as witli 

 ihi' molecules or atoms of ponderable matter. The conditions of^ 

 statistical equilibi-ium l»etween the resonators and the aether may be 

 thoroughly investigated by means of the equations of the electro- 

 magnetic tield. As to the jiarliiittii of energy between the vibrations 

 <»f the resonators and the moleculai- motions in the body, Planck has 

 jiot endeavoured to give an idea of the j)rocesses by which it takes 

 [)lace. He has used other modes of reasoning, of which I shall only 

 mention one, w Inch is to be found in his later papers on the subject and 

 which consists in the determination of that distribution of energy that 

 is tt> be considered as the most |»robable. 1 shall not here discuss the way 

 in which the Jiotion of i)robability is introduced in Planck's theory 

 and which is jiot the only one that may be chosen. It will sullice 

 to mention an assumption that is made about the cpiantities of energy 

 Ihat may l)e gained or lost l)y the resonators. These <piantities are 

 supposed to !)e made u|> of a cei-tai]i number oi' ji/ute j)ortions, 

 whose amount is fixed for every resonator; according to Planck, the 

 energy that is stored up in a resonator cannot increase or diminish 

 l>y gradual changes, but oidy by whole "units «tfenei-gy", as we may 

 call the portions we have just spoken of. Besides, Planck has found 

 it necessary to ascribe to these units a magnitude depending on the 

 fre({uency n of the free vibrations of the resonator, the magnitude 



being represented bv — . 



2jr 



3 

 As to the constant /•, it has a very simjtie physical meaning; — kT 



is the mean kinetic energy of the molecule of a gas at the tempe- 

 rature T. 



1) Planck, Diude"s Ann., Bd. 1, p. 69, 1900: Bd. 4, p.p. 553 and 564, 1901. 



