198 Professor Joseph John Thomson [Jan. 19, 



the pressure is low, sometimes varies so rapidly with the current i as 

 to b^; roughly inversely proportional to it. The following are some 

 values of / and P for a gas at a constant low pressure as the tempera- 

 ture of the platinum strip was increased. The numbers are in the 

 order of increasing temperature : — 



Such a simple relation between P and i is, however, exceptional. 



The fact that the potential differences at which ionization by 

 collision, or luminosity begin depend upon the current density, shows 

 that the ionization or luminosity of an atom need not, and indeed 

 cannot entirely, be the result of a single colUsion between a corpuscle 

 and the atom. For if that were the case, then, since the energy of 

 the corpuscle depends only upon the electric field and not upon the 

 current density, the effect of increasing the current density would 

 merely be to increase in the same proportion the number of luminous 

 atoms, while as a matter of fact if the potential difference is kept 

 constant and the current increased by raising the temperature of the 

 platinum strip, the increase in the luminosity is greater out of all 

 proportion than the increase in the strength of the current. 



The result however, is easily explained if we look at the question 

 from the following point of view. Suppose that for ionization or 

 luminosity to take place the internal energy of the atom must increase 

 by certain amounts, say E^, Eg respectively. Then, if the energy 

 possessed by the corpuscle were very great, the result, of one collision 

 with an atom might be to give to the atom enough energy to ionise 

 it or make it luminous, or both. But eve^i if the corpuscle were less 

 energetic, and did not in one collision give enough internal energy to 

 the atom to ionise it, it would communicate some energy to it ; and if 

 the atom had any power of storing up energy, this would form a 

 contribution towards the critical amount of energy required by the 

 atom ])efore it is ionised. The atom, after having had this energy com- 

 municated to it, would not, as long as it retained any of it, require so 

 much energy to ionise it as before. The atom, too, might acquire 

 energy, not merely by corpuscles striking against itself, but also by 

 the collision of corpuscles with neighbouring atoms. Such collisions 

 generate soft Rontgen rays, the energy of which might be absorbed 

 by the atom under consideration, and help to raise its energy to the 

 critical point. The energy in the Rontgen rays might by itself 

 raise the internal energy of the atom to this value, or else raise it so 

 nearly to this value that the collision with a corjniscle would give it 

 enough energy to carry it ])iist the critical stage The rate at which 

 the energy, due to collisions of corpuscles with itself or with 



