1886.] electric discharge in a uniform electric field. 407 



the negative electrode than at the positive, but there will still 

 however be some decomposed at the positive. The molecules 

 which are decomposed at the positive electrode are those moving 

 away from it, for the molecules which are dissociated are those 

 which are moving along the lines of force, and the lines of force 

 point away from the positive electrode. Since the molecules will 

 have to move some distance before they are decomposed, the 

 decomposition will take place some little way from the positive 

 electrode ; the explosion produced by the discharge will drive the 

 dissociated molecules in both directions, so that some will be 

 driven towards the positive electrode and will recombine them, 

 so that if the distance which a molecule has to travel before it 

 is decomposed be not too great, the luminous glow will extend 

 right up to the positive electrode. If the field be uniform this 

 glow will stretch away from the positive electrode, and we shall 

 have the glow which we observed in the experiments with parallel 

 plates when we used electrically weak gases. 



The very interesting experiments of Dr Schuster on the electric 

 discharge through mercury vapour which is monatomic, and there- 

 fore cannot be dissociated, shews that though in this case the 

 electric discharge presents quite different characteristics yet it 

 still exists. The electric field is still discharged, though with great 

 difficulty, so that there must be other means of electric discharge 

 besides that of the decomposition of the molecules. 



What is essential for the electric discharge is that there should 

 be some means of dissipating the energy of the electric field ; 

 the decomposition of the molecules is one way, and we have seen 

 that there is considerable evidence that part at least of the energy 

 in the electric field is dissipated through this channel, the fact 

 that the discharge takes place through mercury vapour shews 

 that there must be other means of dissipating the energy. The 

 one that seems most obviously suggested by what is perhaps the 

 most striking feature in the discharge, the luminosity, is that the 

 energy is dissipated by radiation. We shall consider some of the 

 consequences of supposing that this is the case. When a system 

 of molecules is acted on by any system of forces, the kinetic theory 

 of gases tells us that the distribution of kinetic energy among the 

 molecules is exactly the same as if no forces acted upon the 

 molecules provided the temperature is the same. So that the 

 only effect which could be produced by the application of a system 

 of forces to a collection of molecules is one that would shew itself 

 as a rise in temperature, the distribution of kinetic energy being 

 a function merely of the temperature and not of the applied forces. 

 Now the radiation presumably depends upon the distribution of 



