In 1935 Hideki Yukawa postulated that the nucleus of the atom was held together by a strong force that over powered the electromagnetic repulsion of the protons. He further postulated that a particle was exchanged between the protons and neutrons to cause this force. Calculating an approximate mass for the particle of about 100 MeV he called it a meson. Twelve year later Giuseppe Occhialini discovered the pi meson with a mass of 146 MeV.
—David Foss