33 Presidential Address 



served fact that the energy is really shared 

 into only a small number of equal parts. 

 For if vibration-possibilities have to be 

 taken into account, the number of de- 

 grees of molecular freedom must be very 

 large, and energy shared among them 

 ought soon to be all frittered away; 

 whereas it is not. Hence the idea is 

 suggested that minor degrees of freedom 

 are initially excluded from sharing the 

 energy, because they cannot be supplied 

 with less than one atom of it. 



I should prefer to express the fact by 

 saying that the ordinary encounters of 

 molecules are not of a kind able to excite 

 atomic vibrations, or in any way to dis- 

 turb the ether. Spectroscopic or lumin- 

 ous vibrations of an atom are excited only 

 by an exceptionally violent kind of col- 

 lision, which may be spoken of as chemi- 

 cal clash; the ordinary molecular orbital 

 encounters, always going on at the rate 

 of millions a second, are ineffective in 

 that respect, except in the case of phos- 

 phorescent or luminescent substances. 

 That common molecular deflexions are 



