ELEMENTARY PARTICLES OF PHYSICS—ANDERSON 205 
of nuclear energy. These and other investigations all combined to 
prove that the proton, the nucleus of the simplest of all the chemical 
atoms, hydrogen, is a constituent of all other chemical atoms, and 
hence is in fact one of the elementary particles of matter. 
In 1930, then, the physicist had at his disposal two elementary ma- 
terial particles, the electron and the protron, in terms of which to try 
to understand the structure of all matter. In this undertaking the 
physicist realized many great successes, but in many instances his 
efforts resulted in sharp failures. Apparently the world was not to 
be understood in terms as simple as these. 
In general the physicist was successful in understanding those phe- 
nomena which we may classify, for want of a better term, as extra- 
nuclear phenomena, and he was unsuccessful in understanding those 
phenomena which we may classify as nuclear phenomena. By extra- 
nuclear phenomena we mean those processes in which the electrons 
which form the outer shells of the atom are the active participating 
agents; in this type of phenomena the central core of the atom, or the 
nucleus, is present but remains undisturbed and does not participate 
actively. Nuclear phenomena, on the other hand, are those in which 
the nucleus is the active participant. 
Extranuclear phenomena and nuclear phenomena have a great 
many distinguishing characteristics. One of the most interesting and 
important of these distinguishing characteristics is concerned with the 
level of energies involved. Extranuclear phenomena involve very low 
energies as compared with nuclear phenomena. The physicist uses the 
term electron volt as a measure of energy. The energies of extranuclear 
phenomena are found usually to range from a fraction of one electron 
volt to several electron volts, whereas nuclear phenomena are found 
usually to correspond to several millions of electron volts. 
In our environment almost every phenomenon in nature represents 
an extranuclear phenomenon: for example, the burning of coal, the 
growth of plants, the generation of electric power by conventional 
means, the fermentation of wine, the explosion of dynamite, and others 
in uncountable numbers. Nuclear phenomena are not so common- 
place, but a few examples may be mentioned: for example, the gen- 
eration of the sun’s heat, the decay of radium, the manufacture of plu- 
tonium, the absorption of cosmic rays in the earth’s atmosphere, the 
explosion of an atom bomb. 
The concept of energy has been introduced here because of the great 
importance that this concept has in the discussion of any physical phe- 
nomenon. I have stated that extranuclear phenomena represent low- 
energy phenomena and nuclear phenomena represent high-energy 
phenomena. To be more accurate I should have said that in extra- 
nuclear phenomena we find low concentrations of energy; that is, the 
energy changes that one associates with a single elementary particle 
