SOME CONTEMPOR.IRY ADVANCES IN PHYSICS— XI 469 



If the gas or the electron-stream is extremely dense, positive ions 

 appear when the energy of the bombarding electrons is lower than the 

 ionizing-energy as determined by experiments with more rarefied gas 

 or a scantier stream of electrons. Various reasons are assigned for this 

 in various cases; one fundamental reason is, that an atom struck by 

 an electron having less than the ionizing-energy may be put into ab- 

 normal states of some duration, in which it can be ionized by receiving 

 a smaller amount of energy than would ionize it in its normal state. 

 In Fig. 2 the measured values of ionizing-potential are plotted.^ 

 There is a way of expressing these and other yet-to-be-presented 

 facts about ionizing-energies, which at this point will probably seem 



+ so 



ATOMIC NUMBER 



Fig. 2 



unnatural but later will be highly convenient. Suppose that by trans- 

 fer of the ionizing-energy Fo to an atom it is converted into a system 

 composed of an ion bearing charge -f e and a free electron. This sys- 

 tem has potential energy Vq relatively to the normal state of the atom. 

 The detached electron may wander ofif and the ion eventually unite 

 itself with another electron. It is convenient, therefore (whether or 

 not it is strictly legitimate) to think of this potential energy Vq as being 

 associated with the ion alone; and to say that the atom possesses, in 

 addition to its normal state, one or more states of ionization or states of 

 the ionized atom, each of them characterized by a certain value of po- 



5 I am deeply indebted to Professor F. A. Saunders, who has kept a current cata- 

 logue of published values of ionizing-potentials, for enabling me to copy his tab- 

 ulations. 



