208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
The mesotron is a particle some 200 times as massive as an electron, 
and therefore about one-tenth as massive as either a proton or neutron. 
It occurs with both positive and negative electric charge. The dis- 
covery of the mesotron did not come quickly and accidentally as was 
the case with the positron and the neutron. It came only after the 
completion of a sustained series of observations, covering a period of 
4 years, which were designed to remove certain inconsistencies always 
present when we attempted to understand certain ‘cosmic-ray phe- 
nomena in terms of the elementary particles then known. These 
inconsistencies were removed in terms of the existence of the mesotron, 
whose discovery was publicly announced in 1936. 
Unlike the neutron, the mesotron was not a particle to be imme- 
diately welcomed by the physicist. The physicist makes his advances 
by simplifying his understanding of nature; hence, a physical world 
which could be explained in terms of only one or two distinct elemen- 
tary particles would be most to his liking. The discovery of the 
mesotron did not introduce a simplification; rather it complicated the 
situation for it increased the number of material elementary particles 
from four to six. Apparently the Creator does not favor a world of 
too great simplicity. 
Before the discovery of the mesotron a Japanese physicist, Yukawa, 
had postulated on theoretical grounds the possible existence of parti- 
cles of a mass intermediate between a proton and an electron. His 
theory, however, was not generally known to physicists at that time, 
and did not have any part at all in the discovery of the mesotron. 
Had this theory been generally known it is still doubtful if it would 
have affected the course of cosmic-ray research, since, unlike the 
Dirac theory of the positron, it would not have served as so useful a 
guide in pointing out the most fruitful directions for the research to 
follow. 
Like the positron the mesotron has a very short life expectancy. 
In free space, both positive and negative mesotrons have a normal life 
span of just over two-millionths of a second, after which time they 
spontaneously disintegrate. Very recent observations have shown 
that in all probability the spontaneous disintegration of a mesotron 
results in the simultaneous production of an electron and two neutrinos. 
Neutrinos are the interesting elementary particles which had pre- 
viously been invented in order to balance energy and momentum in 
the process in which an electron is produced when a radioactive nucleus 
decays. A similar situation exists in the case of the decay of a 
mesotron except that here, because the mesotron disappears entirely, 
it is necessary to postulate the emission of two neutrinos in order to 
balance energy and momentum. 
In free space mesotrons spontaneously decay after about two- 
millionths of a second. In the presence of matter, a mesotron of 
