135 
1921-22.] On the Quantum Mechanism in the Atom. 
depend on the mass m of the electron, but only on its charge e and on 
quantities depending on the atomic structure with which the electron 
collides. 
The absorbed energy appears in the atom as a magnetic current, 
specified by the angular velocity o) = 2eM/A, so that between the 
absorbed energy and this angular velocity there exists the relation (by 
(8) and (9)) 
U = eMw (11) 
It may, I think, be urged in support of the above explanation of 
quantum absorption of energy that it postulates no structure in the atom 
beyond one of a kind which other investigators have introduced in order to 
account for a totally different class of phenomena, namely, those of induced 
magnetisation. If on grounds connected with induced magnetisation we 
have come to accept the existence of a structure within the atom capable 
of providing what is here called a magnetic current — and it is difficult to 
see how induced paramagnetism can be explained without some structure 
which will do this, — then the whole argument of the present section seems 
to be forced on us inevitably : and, as we have seen, it entails the neces- 
sity that exchanges of energy between atoms and electrons must be of 
quantum type. 
At the same time, we must bear in mind that the function of models is 
merely to suggest the correct differential equations of the phenomenon : 
when the differential equations have been obtained, the model may be 
discarded. Instances of this in the history of physics are abundant : to 
name only the most famous of them, it was a model of rolling particles, 
idle wheels, and cellular vortices that suggested to Maxwell the correct 
differential equations of the aether. The model, having served its purpose, 
soon dropped out of sight : and the aether itself appears to be following it 
into oblivion. The differential equations alone remain, and in the hands 
of the relativists have provided the foundation for a complete reconstruc- 
tion of our ideas of the universe. 
We may therefore be well satisfied if the above mathematical equations 
represent correctly the interaction between an atom and an electron which 
is approaching it with a velocity not great enough to ionise it, but great 
enough to evoke a single-line spectrum : * the ‘‘ magnetic structure ” which 
suggested the equations need not be insisted on. 
* The case when the electron approaches with velocity great enough to ionise the atom 
would correspond in our model to the case when the electron imparts to the magnetic 
structure so great an angular velocity that the structure explodes under the centrifugal 
strain. 
