QUANTUM THEORY 
57 
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By way of contrast, consider what happens when an atom emits an 
electron instead of a quantum of radiation. If an electron encounters a 
normal atom there is a certain probability that the atom will be ionised 
so that two electrons (the original electron and an additional one) leave 
the atom. The converse process occurs when two electrons meet an ionised 
atom simultaneously and one of them (which would have escaped if it 
had been alone) is captured through the confusion caused by the entrance 
of the other. In that case we apply Law I, and deduce that the ionisations 
by electronic impact must be balanced by the captures due to the combined 
encounter of two electrons simultaneously. 
This striking difference of treatment of electrons and radiation is 
justified experimentally. It strongly suggests that free radiation has no 
atomicity of constitution. If it consisted of independent atoms it would 
scarcely be possible to avoid effects due to simultaneous action of two 
atoms of radiation, the frequency of such effects being proportional to 
the square of the intensity. It is not sufficient to suppose that these 
combined effects are too small to be observed; in Einstein’s equation the 
place which should have been theirs is definitely assigned by default to 
other agencies. 
The modern quantum theory appears to incline to this view that free 
radiation is continuous, and that the quantum is only called into being 
in the process of interaction of radiation and matter. 
As the general conception of the quantum theory has undergone some 
modification in recent years, it will be well to indicate how it is now re 
garded. We start with an electromagnetic field to which Maxwell’s equa 
tions rigorously apply. This is the tensor F of the relativity theory or 
(A, Y, Z, a, ft, y) of the classical theory; and Maxwell’s equations assert 
( 1 ) that it is the curl of an electromagnetic potential, ( 2 ) that its divergence 
is the electric charge and current vector. By Maxwell’s theory disturbances 
of this vector are electromagnetic waves propagated with the fundamental 
velocity c, and showing the phenomena of interference, diffraction, etc. 
in accordance with the undulatory theory. There is no discontinuity or 
quantum structure involved in this field. We have next to consider how 
the field and its waves become amenable to experimental detection— 
nothing having been yet said on this point. The detection is consequent 
on energy-changes provoked in material systems. The electric and magnetic 
forces are not in themselves observable; the observable effects arise from 
the mechanical or ponderomotive force of the field which is represented 
by another vector (F^F™ in the relativity theory). The simplest state 
ment on the classical theory of the observable effects arising from the 
electromagnetic field is that it involves a flux of energy measured by the 
Poynting vector, or vector-product of the electric and magnetic forces, 
together with a flux of momentum represented by the well-known Max-