244 
THE QUANTUM [xvm. i 
a large number of instances, or when we regard them as results 
which follow from some theory which is known to embrace many 
facts of observation or experiment. 
“ Explanation of a fact can mean no more than this, correla 
tion with and co-ordination among an existing body of other 
facts which can all be similarly related to the same general 
principle. This, however, is enough ” (R. H. Fowler). 
This method of answering questions by showing that they may 
be included in a more general category—a method at least as 
old as our oldest literature—is well illustrated by Clerk Maxwell’s 
“ explanation ” of light as an electromagnetic disturbance 
propagated in a medium possessing certain characteristic electric 
and magnetic properties. Here we are no longer passing from 
the unknown to the familiar, we might even be said to be turning 
from a region which is well-known to one which is recondite and 
strange. But by taking this step we are able to secure a more 
comprehensive theory which includes under one head two types 
of phenomena—light and electromagnetic waves—which had 
previously been considered distinct. The opposition felt by 
certain physicists, including Lord Kelvin, to the electromagnetic 
theory of light is reflected in the Preface to the first edition of 
Schuster’s Optics, 1904, which begins with the words : “ There 
is at present no theory of Optics in the sense that the elastic 
solid theory was accepted fifty years ago. We have aban 
doned that theory, and learned that the undulations of light 
are electromagnetic waves differing only in linear dimensions 
from the disturbances which are generated by oscillating electric 
currents or moving magnets. But so long as the character of 
the displacements which constitute the waves remains undefined 
we cannot pretend to have established a theory of light.” 
The view that the real nature of phenomena will never be 
understood led Wilhelm Ostwald, who for a long period withheld 
his allegiance from the atomic theory, to his philosophy of 
“ Energetics,” but even Ostwald in his later years was compelled 
to avow his belief in the discrete or grained structure of matter. 
“ The isolation and counting of gaseous ions on the one hand 
. . . and on the other the agreement of the Brownian move 
ments with the requirements of the kinetic hypothesis . . . 
justify the most cautious scientist in now speaking of the experi 
mental proof of the atomic theory of matter. The atomic 
hypothesis is thus raised to the position of a scientifically well- 
founded theory.” 
Poincar6 also in his last scientific book, was forced to the 
same conclusion. “ The atoms are no longer a convenient 
fiction ; it seems to us that we can, so to speak, see them, since 
we know how to count them.”