184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



it appears as energy released. A graphic picture of what happens 

 may be made thus : 



Figure 3. 



The mass lost in the change from uranium to barium represented by 

 PQ is 100 times the mass lost in the change from hydrogen to helium 

 represented by RS. 



The same facts may be stated in terms of energy. The "binding 

 energy" of a nucleus is defined to be the difference between the sum of 

 the masses of all the protons and neutrons which went into its compo- 

 sition and the true nuclear mass. Thus the elements in the middle of 

 the periodic table have the greatest binding energy or are most 

 strongly bound. To break them up energy would have to be supplied ; 

 but when atoms of elements near either end of the table are transformed 

 energy is released. 



A CHAIN REACTION 



Ordinary combustion is an illustration of a chain reaction. We 

 light a match. It sets fire to some paper. This ignites the kindling 

 next it. This in turn, we hope, will ignite the coal. It is self- 

 sustaining. The fact that more than one neutron came out as a fission 

 product, when only one caused the fission of U^^^, suggested the possibil- 

 ity of a chain reaction, but no one knew whether or not it would work. 

 If from 1 neutron came 3, from 3, 9, from 9, 27, the reaction would 

 expand at a terrific rate. But it should be remembered that it is the 

 rare isotope U^^* which is broken by the neutron, and this occurs in 

 nature mixed with 139 times as much U^^. So at least four things could 

 happen to the emitted neutrons. They might (1) escape, (2) be 



