PHOSPHORESCENT MOLECULES 289 



the presence of sucli large luminous molecules will 

 greatly facilitate the passage of the discharge. There 

 is very strong evidence that luminous molecules in 

 a gas do produce ionisation, and speaking generally, in 

 the positive column in an ordinary electric discharge 

 through a rarefied gas, namely, the part of the dis- 

 charge extending from the positive electrode to close 

 upon the negative electrode where a dark space 

 exists, the electromotive force is comparatively small, 

 the greater part of the force needed to produce the 

 discharge being required in the neighbourhood of 

 the negative electrode. Throughout the positive 

 column the force is much too small to produce ionisa- 

 tion by collisions between ions and molecules, when 

 the former are acted upon by the electric field. The 

 fall of potential, that is, the electric force in 

 the positive column, is from 5 to 30 volts per 

 centimetre, whereas a force of between 200 and 300 

 volts per centimetre is usually the minimum to 

 produce a discharge. It is thus probable that the 

 ionisation in the positive column results from some 

 kind of radiation from the molecules themselves. 

 It is likewise not improbable that the ionisation 

 which accompanies the phosphorescent after-glow is 

 d^ie to the radiation from the glowing molecules 

 when they come very close to other molecules, 

 that is, when they collide with them. It is there- 

 fore not merely the collision between ions, but 

 those between luminous molecules that should be 

 taken into account. 



In fact, molecular theory in its relation to the 



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