ELEMENTARY PARTICLES OF PHYSICS—ANDERSON 209 
negative charge may terminate its existence in an even shorter time. 
It does this by entering an atomic nucleus or, in the language of the 
physicist, by undergoing nuclear capture. 
The mesotrons observed in cosmic rays are produced by the very 
high energy particles of the primary cosmic-ray beam as it comes into 
the earth from outer space and plunges through the earth’s atmos- 
phere. In a manner somewhat analogous to the creation of positrons 
and electrons, the mesotrons are born out of the tremendous energies 
carried by the primary cosmic-ray beam. 
There are many interesting phenomena involved in the birth and 
death of mesotrons and in the violent nuclear processes which accom- 
pany these phenomena, but it will not be possible to discuss them here. 
However, I should like to mention in this connection two important 
advances which have been made within the last 2 years. 
RECENT ADVANCES IN NUCLEAR RESEARCH 
One of these is the work under way in Bristol, England, by Powell 
and his coworkers, which has consisted of a detailed analysis of the 
tracks produced by mesotrons in the emulsions of photographic plates. 
These investigators have discovered a mesotron of a new type which is 
heavier than the ordinary mesotron. It is about 285 times as massive 
as an electron, whereas the ordinary mesotron is about 215 times as 
heavy. The heavy mesotron has only a very short life; it lives only 
about one one-hundredth as long as the light mesotron, after which 
time it disintegrates and produces a light-weight mesotron and another 
particle which is probably a neutrino. The negatively charged heavy- 
weight mesotron may also directly enter an atomic nucleus and give 
rise to a violent nuclear disruption. 
Although both the newly discovered heavy mesotrons and the light 
mesotrons discovered in 1936 have some properties in common—e. g., 
both types of particles occur with positive and negative charges, both 
have short lives, and both are found in cosmic rays—nevertheless in 
some very fundamental respects they are entirely different types of 
elementary particles. The heavy mesotron interacts very strongly 
with atomic nuclei, but the light mesotron interacts only very weakly 
with atomic nuclei. Another difference lies in the respective values of 
that important property known as the spin or angular momentum; 
recent researches indicate that the heavy mesotron has an integral 
spin, whereas the light mesotron has a half-integral spin. 
In all probability it is the heavy mesotron and not the light mesotron 
which is to be identified with the particle first postulated on theoretical 
grounds by Yukawa in 1934. The theory of Yukawa even in its pres- 
ent state today is very primitive. However, this theory still provides 
the best basic concept in terms of which to understand processes in- 
volving mesotrons, and after further development in the future the 
