1895.] Motions of and within Molecules, $c. 179 



illuminated. In fact many such bodies are evanescently phosphor- 

 escent,* and the survival of the phosphorescence when the stimu- 

 lating light has been withdrawn can be observed, and its duration 

 measured, by the phosphoroscope. Although in most cases its dura- 

 tion is but a small fraction of a second, nevertheless in every case 

 that has been observed, indeed in every case that can be observed 

 with the phosphoroscope, it is a duration of immense length com- 

 pared with the almost inconceivable rapidity of molecular events, in 

 comparison with which even the thousandth part of one second is a 

 vastly long period of time.f 



In all such bodies therefore there are events of the class Bb. 



Solids only seem to have been examined by the phosphoroscope. 

 But we may feel assured that the same dynamical conditions prevail, 

 certainly in liquids and probably in gases. 



Let ns next consider what bearing this has on the interpretation 

 which is to be put on a high ratio of the two specific heats. 



An event of the Bb class, which subsides so rapidly as to require 

 the phosphoroscope to detect it, will behave, in any protracted 

 experiment for determining the ratio of the two specific heats, in the 

 same way as events of the Ba class. But this ceases to be the case 

 where the ratio of the two specific heats is determined by experi- 

 ments on sound ; and in all the experiments which have been 

 made use of it ceases to be the case when Bb events are as slow in 

 subsiding as some of them are in conspicuously phosphorescent bodies. 



Now the method by which the ratio of the two specific beats has 

 been determined for argon and helium has been by experiments on 

 sound ; and as the value furnished by this method depends upon Ba 

 events, it is competent to supply information about them only. It 

 gives no information as to the energy involved in events of the Bb 

 class. Accordingly it remains quite possible that Bb events may be 

 easily evoked by stimulation of argon and helium, and that while in 

 existence they may engross a considerable share of the total energy 

 in the gas. That this is the case would seem to be evidenced by the 

 vivid spectra which these gases exhibit under the influence of elec- 

 tricity. 



* The colours of objects, when not mere interference phenomena, are due to the 

 excitation of Ba or Bb events within the molecules by certain rays of the incident 

 light. In both cases the acting rays yield up their energy ; but when Ba events 

 take their place, the body is simply warmed : when it is Bb events that come into 

 existence, the body for a short time subsequently radiates light. In the one case 

 the colour of the body is that of incident light which is not absorbed; in the 

 other case it is in general the colour of incident light which is absorbed. Of course 

 if both causes are in operation they produce their conjoint effect. A few out- 

 lying cases, such as that of fluorescence, require a slightly modified treatment. 



t The thousandth of a second bears about the same relation to molecular events 

 that 10,000 years does to the motions of the limbs of animals. 



