THE SMALLEST PARTICLES OF MATTER 33 



nuclei and produce further fission, and (b) the speed with which 

 the material of the bomb flies apart. Since a mass of plutonium 

 or of U 235 above a critical size would spontaneously undergo 

 explosive fission, to produce satisfactory detonation the bomb 

 must consist of separate pieces so small that they are incapable of 

 doing this. These are shot together to create the explosion. 



This "critical size factor" is one of the most amazing properties 

 of fissionable materials. The "critical mass" necessary for explo- 

 sion is guessed to be from five to fifteen pounds. 



The Successive Levels of Material Structure 



The accompanying table shows the various material units at 

 present recognized, and their position in order of size and mass 

 relative to units which are generally known. It must not be 

 assumed, however, that our present inability to break up the 

 "ultimate" units, e.g., protons, electrons, etc. represented on the 

 diagram as of zero order of complexity, is to be considered as final 

 evidence of their actual simplicity. In fact, the "spin" of an elec- 

 tron is a factor well recognized in atomic and nuclear physics, and 

 this may be due to some kind of complexity. Besides, we cannot, 

 even in imagination, go down to the ultimate particle or particles 

 of matter, even though we recognize the realities which emerge 

 from them. 



In order to locate man and biological happenings in their rela- 

 tive position in universe, especially introducing the essential time 

 factor so often omitted from consideration, there is also given a 

 table showing measurements in space, mass, and time, reaching in 

 each case from the greatest to the smallest known or readily 

 calculable "unit." 



Dr. Edwin Hubble (Mt. Wilson Observatory, Carnegie Institu- 

 tion) compares our stellar system to a drifting swarm of bees, the 

 sun, with its relatively insignificant family of planets, being a fairly 

 small star in the swarm. Astronomers, peering out into the vast 

 emptiness of space, see other stellar systems so distant that they 

 appear merely as unresolvable patches of light, and are therefore 

 called nebulas. The "observable region" is a sphere, roughly 

 600 million light-years in diameter, within which are scattered 

 about 100 million nebulas, averaging 1| million light-years apart, 

 80 million times brighter than the sun, and 800 million times 

 greater in mass. The faintest nebulas that our present telescopes 

 can detect lie at the peripheral surface of this sphere. Whatever 



