246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 6 



called protons, and uncharged particles called neutrons. Around 

 this core we have a cloud of negative electrons. 



Hydrogen, the lightest of the atoms, has in its normal state only one 

 proton in its core or nucleus, and attendant upon this is a single 

 electron. Helium has two protons and two neutrons in its core. 

 Uranium, until recently the heaviest element known, has, in its 

 nucleus, 92 protons. In the case of one kind of radium there are 

 143 neutrons, while in another kind there are 146 neutrons. It is 

 the number of protons that determines the chemical nature of the 

 element. Atoms having the same number of protons but different 

 numbers of neutrons are called isotopes of one another. 



Subsidiary elementary particles. — Although neutrons, protons, and 

 negative electrons form the only permanent constituents of the atom, 

 other particles come into existence during periods of drastic perturba- 

 tion such as occur when a primary cosmic ray or one of its descendants 

 strikes an atom of air. 



First, we have a particle called the positron, which is the counter- 

 part of the negative electron, having the same mass, but carrying an 

 electric charge equal and opposite to that of the negative electron. 

 Sometimes it is called a positive electron. 



Other important particles are the mesotrons. The mesotrons are 

 peculiar in the sense that they have but a finite life and die in due 

 course without the intervention of any external agency. 



When the mesotrons were first discovered, it was thought that they 

 were all of one kind, but as knowledge has advanced, it now appears 

 that there are several kinds, which are in part related to one another 

 like child and parent, or brother and sister. When a proton of high 

 energy enters our atmosphere, collisions with the atoms of the air re- 

 sult in the proton's disintegration and the formation of mesotrons 

 which are, as it were, born from its ashes. When a heavy atom enters 

 the atmosphere, its individual protons suffer a similar fate, with the 

 resulting production of mesotrons. The neutrons of the atomic nuclei 

 seem to be preserved from a like fate, but they are not completely im- 

 mune, because a neutron freed from its home in an atomic nucleus 

 sacrifices its right of permanent existence and dies after a period of 

 about 20 minutes. 



On page 249 we shall return to a fuller discussion of the individuals 

 of the mesotron family. 



In phenomena of the kind we are discussing, another particle has 

 entered the picture, the neutrino. Its presence has only been inferred, 

 but never observed in the ordinary sense of the word. As is well 

 known, physicists place great faith in the conservation of energy and 

 momentum in atomic processes. However, it appears that in some of 

 these processes, adding up all the contributions of the various particles 



