210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
Yukawa theory may possibly provide an understanding in terms of 
mesotron exchange forces of that all-important problem as to the 
nature of the forces acting between the particles inside a nucleus. So 
far no satisfactory theory has been developed in terms of which to 
understand many of even the simplest phenomena involving the 
nucleus. To acquire a quantitative understanding of the interactions 
of the elementary particles of matter and of the fundamental nuclear 
processes is one of the great tasks of theoretical physics today. 
To complete our list of elementary particles we should also include 
the photon. This particle, together with the neutrino as noted above, 
is, however, in a somewhat different category from the other types 
of particles. The photon is not a material particle, in the sense that 
it cannot be identified with any particle which can exist at rest and 
have associated with it a finite amount of ponderable material sub- 
stance. Photons are to be identified only with radiation or radiant 
energy. ‘The neutrino must also be placed in a special category, since 
it cannot have associated with it an appreciable amount of ponderable 
material substance if any at all, and since it has never been directly 
observed. 
Tn all, then, the physicist at the present time recognizes at least 10 
distinct elementary particles of matter. Whether this list is complete 
or not no one can say with certainty. The indications are that the 
list is not complete, for evidence seems to be rapidly accumulating for 
the existence of at least one additional elementary particle. This 
particle is found in cosmic rays and appears to have a mass some 1,000 
times the mass of the electron. But what its properties are and how 
it is related to the light and heavy mesotrons and to the other elemen- 
tary particles of matter is a subject which must await the results of 
further observations. 
The thought of probable further additions to the list of elementary 
particles of matter suggests a question which is quite apart from 
physics and has to do simply with the naming of new particles. We 
have here actually an interesting example of the great difficulties that 
physicists sometimes have merely in assigning labels or names to the 
various concepts which their experience or their theories have brought 
forth. It is usually necessary to choose some sort of name for these 
concepts, whether they be elementary particles of matter or something 
else, at a time before all the facts regarding them are known. In 1937 
the term mesotron was suggested to designate the new particle of 
intermediate mass discovered in the cosmic rays in 1936. Since then 
this term has often been contracted to meson and has been so em- 
ployed. Since the discovery of the new particle whose mass is greater 
than the mass of the original cosmic-ray mesotron, the term mesotron 
or meson has been employed to designate both types of particles and 
the Greek-letter prefixes 7 and yp used to differentiate between them. 
