

Kinetic Theory of Gas. 381 



vapour, and are uninfluenced by encounters between air 

 molecules and vapour molecules. Hence they are due to 

 something different from the mere kinetic energy of the col- 

 lisions ; and this something may be, and probably is, that 

 during the struggle between molecules of the same kind, a 

 struggle which is a protracted struggle from the molecular 

 standpoint, there occurs either always, or now and then, an 

 interchange of some of the chemical atoms constituting the 

 molecules. This, which is equivalent to two chemical decom- 

 positions, followed by two equivalent chemical combinations, 

 must set the electrons concerned into a state of more or less 

 activity. 



This interchange of atoms during the encounters is pre- 

 sumably the source, not only of such absorption-spectra as 

 that of chlorochromic anhydride, but also of the bright 

 spectra seen in Pliicker's tubes. 



If the gas be monatomic, the spectrum is probably emitted 

 only when the circumstances are such that two molecules can 

 temporarily coalesce into a diatomic molecule during the 

 encounter, and become dissociated when the encounter is 

 over ; but in all cases where there is no ultimate change in 

 the chemical constitution of the gas, its spectrum seems to be 

 due to some event which is equivalent to equal and opposite 

 chemical reactions having taken place during either all or 

 some of the encounters. 



An excellent way of helping us to appreciate the events 

 with which we have to deal in molecular physics, is to con- 

 ceive a model of them in which the durations shall all be 

 enlarged 600 billions of times (6xl0 14 ). This particular 

 magnification is found to have special convenience attached 

 to it*. If prolonged to this extent, the most rapidly recur- 

 ring motions in nature that are as yet known to us, viz., 

 those periodic events in a gas which give rise to the lines in 

 its spectrum, would swing at rates comparable with the motions 



probable explanation has been suggested to me by Professor FitzGerald. 

 It is that the electrons are sufficiently linked together through the aether 

 to be but little affected by the A and Ba events while on their side com- 

 petent to influence such events, in the way described on p. 376. Under 

 these circumstances the number of encounters might be increased by 

 adding air, or otherwise, without perceptible effect on them ; while they, 

 if susceptible of being excited by light in the way that phosphorescent 

 bodies are, would act as carriers of energy from the aether to the A and 

 ~Ba events, thus causing absorption of light. It ought not to be imprac- 

 ticable to devise experiments which will determine with certainty which 

 of the possible explanations is the real one.] 



* Wave-lengths of rays of light are usually expressed as fractions of a 

 micron, and pendulums beating the same fractions of a second represent 

 the corresponding aBtherial vibrations on the scale employed in the text. 



