130 Proceedings of the Poyal Society of Edinburgh. [Sess. 
§ 2. The Nature of the Interaction between an Atom 
AND AN Approaching Electron. 
Let us now consider an electron approaching an atom with a velocity 
which is not great enough to ionise the atom, but which may- be great 
enough to evoke a '' single-line spectrum.” We shall speak of the encounter 
as a “ collision,” understanding thereby not any actual contact of the two 
bodies, but simply that when they are close to each other there are forces 
between them which may involve a considerable exchange of energy. 
The experimental results indicate that the electron, as it approaches, 
experiences a repulsion which is sufficient to turn it back altogether if its 
kinetic energy is less than liv. We have first to determine the nature of 
this repulsion. Now there are two kinds of force which are capable of acting 
on an electron, namely, electric force and magnetic force, and we have to 
decide which of these two is operative in the present case. The decision 
is easy, since we know that motion through a field of electric force affects 
the kinetic energy of an electron, while motion through a field of magnetic 
force deflects its direction of motion without altering its energy : and in 
our case the fate of the electron depends entirely on whether it possesses 
enough kinetic energy to force its way through the field. The field must 
therefore be electric : that is to say, an electron which is approaching an 
atom experiences, in the vicinity of the atom, a field of electric force. 
We have now to consider whether this field of electric force is always 
present in the vicinity of the atom, whether the electron is there or not, 
or whether the field is evoked in some way by the approach of the atom. 
Here again the decision is not in doubt : the atom cannot maintain a 
permanent electric field in its vicinity unless it either contains an excess 
of electricity of one sign, or is permanently polarised electrically : both of 
which suppositions can be ruled out, since the atoms considered are neutral 
atoms which do not respond to an ordinary electric field. We thus infer 
that the electric field about the atom is not jjermanent, hut is evoked by the 
approach of the electron. 
We have now to consider what kind of mechanism within the atom 
provides this responsive electric field when the electron approaches. 
A first attempt at an explanation might be made by supposing that 
there are within the atom elements which behave like conducting surfaces : 
for if an electric charge is brought into the neighbourhood of a conducting 
surface, the portions of the surface nearest and furthest from the charge 
become oppositely electrified, and an electric field is thus set up. Any 
explanation of this type must, however, be abandoned by reason of the fact 
