PRESIDENT’S ADDRESS. HH 
the energy of a group of colliding molecules must in the long run be 
equally shared among all their degrees of freedom, with the observed 
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 degrees 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 vibra- 
tions, or in any way to disturb the ether. Spectroscopic or luminous 
vibrations of an atom are excited only by an exceptionally violent kind 
of collision, which may be spoken of as chemical 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 ineffective is certain, else all the energy would be dissi- 
pated or transferred from matter into the ether; and the reasonableness 
of their radiative inefficiency is not far to seek, when we consider the 
comparatively leisurely character of molecular movements, at speeds 
comparable with the velocity of sound. Admittedly, however, the 
effective rigidity of molecules must be complete, otherwise the sharing 
of energy must ultimately occur. They do not seem able to be set 
Vibrating by anything less than a certain minimum stimulus; and that 
is the basis for the theory of quanta. 
Quantitative applications of Planck’s theory, to elucidate the other- 
wise shaky stability of the astronomically constituted atom, have been 
made; and the agreement between results so calculated and those 
observed, including a determination of series of spectrum lines, is very 
remarkable. One of the latest contributions to this subject is a paper 
by Dr. Bohr in the ‘ Philosophical Magazine ’ for July this year. 
To show that I am not exaggerating the modern tendency towards 
discontinuity, I quote, from M. Poincaré’s ‘ Derniéres Pensées,’ a 
proposition which he announces in italics as representing a form of 
Professor Planck’s view of which he apparently approves :— 
“A physical system is susceptible of a finite number only of 
distinct conditions; it jumps from one of these conditions to 
another without passing through a continuous series of inter- 
mediate conditions.’ 
Also this from Sir Joseph Larmor’s Preface to Poincaré’s ‘ Science 
and Hypothesis ’:— 
* Still more recently it has been found that the good Bishop 
1913. c 
