﻿456 Sir J. J. Thomson on Ionization 



strike against the molecules of the body, the character of the 

 radiation emitted when the ions recombine will be comparable 

 with that from a body at such a temperature that the average 

 energy of one of its molecules is that due to the fall of the 

 atomic charge through 10 volts. At the temperature 0° C. 

 the average energy of a molecule of a gas is that due to the 

 fall of the atomic charge through 1/30 of a volt, which is 

 1/300 part of the energy of our corpuscles ; the absolute 

 temperature of a body whose molecules have energy equal to 

 the corpuscle is thus 300 x 273, or about 80,000 degrees 

 absolute : this is about 13 times the absolute temperature of 

 ihe sun, and as the maximum energy in the solar spectrum 

 is in the neighbourhood of wave-length 4*9 x 10 -5 cm., and as 

 the wave-length for maximum energy is inversely proportional 

 to the absolute temperature, the maximum energy for light 

 analogous in its properties to the radiation emitted by the 

 recombination of the ions would be in the neighbourhood of 

 wave-length 3*8 X 10~ 6 cm., and would thus correspond to 

 light very far in the ultra-violet. The theory is supported by 

 ihe fact that those regions in a discharge-tube where recom- 

 bination is taking place, such as the negative glow, are the 

 sources of a type of radiation called "Entladungstrahlen " 

 which, has many of the properties of ultra-violet light. 



Suppose now that besides the corpuscles which can be 

 dislodged from the atom by the expenditure of an amount of 

 energy measured by a few volts, there are in the atom other 

 systems which require for their dissociation a much greater 

 amount of energy, say an amount measured by thousands of 

 volts. If the ionization of these systems is to be effected by 

 cathode rays, it will not begin unless the rays have the 

 velocity due to this number of volts ; when, however, the 

 rays have a greater velocity than this, some of these systems 

 will be ionized, and a positive residue produced which will 

 have with respect to a corpuscle at some distance away 

 potential energy measured by x thousand volts. When the 

 residue and the corpuscle unite and reproduce the original 

 system, this amount of energy will have to be radiated away; 

 and the type of the radiation will be such that it is repre- 

 sented by waves with wave-lengths less than those in the 

 preceding case in the proportion of 1 : 100 ,v. Thus the 

 maximum energy would be in the neighbourhood of wave- 

 length 10" 8 cm., i. e. the radiation would be Eontgen radia- 

 tion. Thus the presence in the a^om of systems requiring a 

 large quantity of energy to ionize them, would give rise to 

 Rontgen radiation of a type determined solely hy the pro- 

 perties of the atom. The energy in this radiation would be 



