ELEMENTARY PARTICLES OF PHYSICS—ANDERSON 911 
Thus the term r-mesotron or r-meson designates the heavier particle 
and y-mesotron or p-meson designates the lighter particle. This 
nomenclature seemed satisfactory for a time until continued experi- 
mentation began to show more and more clearly the important basic 
differences between the two types of particles. It is beginning to be 
quite apparent now that the properties of these two types of particles 
are such that they will not naturally fall into the same classification. 
Thus the use of a common generic term, such as mesotron or meson, 
to designate both these types of particles may in the future prove to 
be quite inconvenient and illogical. Just what should be done with 
respect to nomenclature at this time is not clear, but it is a matter 
which should receive very serious consideration, especially in view 
of the apparent entry of still another new elementary particle into 
the fold. 
Another important advance that I want to mention is the recent 
success in producing mesotrons in the large cyclotron on the University 
of California campus at Berkeley. This represents the first time that 
it has been possible by artificial or laboratory methods to imbue a 
single particle of matter with an energy sufficiently high to make pos- 
sible the creation of mesotrons. This they have succeeded in doing 
in Berkeley with their beam of a-particles, or helium nuclei, which 
have been accelerated to an energy of 400 million electron volts. 
They observed the production of both the heavy and light mesotrons, 
and all indications are that the mesotrons thus produced are identical 
with those previously observed among the particles produced by 
the cosmic rays. 
Now in the design stage are other particle-accelerating machines 
which will yield particle energies several times the 400 million electron 
volts so far achieved in the Berkeley cyclotron. When these machines 
are in operation, working at energies up to 6 or 7 billion electron volts, 
we can expect to learn much more about mesotrons and the other ele- 
mentary particles of matter. Moreover, we must expect that a con- 
tinuation of research in cosmic rays will also extend our knowledge 
in this field, since in the cosmic rays particles are available for study 
whose energies are even 10 to 100,000 times greater than those to be 
expected from any of the accelerators that are being planned. 
In conclusion I should like to indicate the possible significance of 
these new discoveries to science and to the world at large. 
In this discussion I have classified physical phenomena, according 
to the energy associated with them, into three categories: (1) low- 
energy or extranuclear phenomena, (2) high-energy or nuclear phe- 
nomena, and (3) extremely high-energy or what we might call, for 
want of a better name, elementary-particle phenomena. Knowledge 
of the first of these, low-energy or extranuclear phenomena, has already 
profoundly affected the life of nearly every human being on earth. 
