Kinetic Theory of Gas. 377 



easily perceive it. We may call the first Bb x events. These 

 are they which can effect the transfer through some few 

 millions of encounters. And we may call the still more 

 isolated events Bb 2 events. These latter, for example, mani- 

 fest their existence when phosphorescence lasts for a whole 

 second or more — truly enormous durations as regards mole- 

 cular activities. 



When an electron is associated with Ba events it will 

 promptly transfer over any excess of energy it receives from 

 the aether to u, v, and w, the translational velocities of the 

 molecule. Accordingly, on the one hand, the temperatnre 

 and pressure of the gas will increase, on the other the setherial 

 undulations that acted on the electron will have ceased — in 

 other words, the gas is one that has an absorption-spectrum. 



If, at the other extreme, the electron is associated with Be, 

 or absolutely isolated events, a beam of light passing through 

 the gas will, if it contain certain rays, set these electrons 

 moving. They, however, will not impart any of their acquired 

 energy to other events going on in the gas, but will continue 

 swinging in all the molecules in coincidence with the electro- 

 magnetic wave as it sweeps past them in the aether ; thus 

 restoring to the latter the same amount of energy which they 

 received, and in the form of an undulation travelling at the 

 same rate, and in the same direction, and oscillating in the 

 same periodic time. Thus, so far as regards the motion of 

 this electron, the gas is transparent. 



Between these extremes electrons associated with Bb events 

 will lie, and may produce any intermediate event. 



It is very instructive to glance over the determinations of 

 7 in the table on p. 374. Take, for example, the diatomic 

 gases. Six of them are transparent, and the other four are 

 coloured. The transparent gases furnish values for y which 

 all lie in the neighbourhood of 1*4, which is the value which 

 would correspond to a molecule having five degrees of freedom 

 if the conditions of Boltzmann's theorem were completely 

 fulfilled. In transparent gases they are probably not much 

 interfered with, since in them the electrons are associated 

 with Bb 2 events, and it is only after many millions of encoun- 

 ters that the aether can, through them, sensibly affect the A 

 and Ba events with which the theorem is concerned. On the 

 other hand, in the coloured gases one or more electrons (in 

 these special gases probably something like six or eight elec- 

 trons, judging from their spectra) are associated with Bb t 

 events, and largely affect the events with which the theorem 

 is concerned. They accordingly cause y to be different from 

 what it would be if the dynamical interactions stood alone, 



