142 THE PATH OF SCIENCE 



energy produced from Einstein's equation (Chapter V, p. 

 115), stating that the energy produced, in ergs, is the change 

 of mass, in grams, multiplied by the square of the velocity of 

 light, which has the tremendous value of 9 X 10-^. When 

 atoms are disintegrated in this way, enormous amounts of 

 energy are released. No effective energy could be obtained 

 from such experiments, ho^\ ever, because only a very few of 

 the charged protons are captured by the lithium nuclei, and 

 so much energy is required to produce the beam of charged 

 protons that the procedure is quite hopeless as a means of 

 producing useful energy. 



What is needed is a nuclear reaction that would be self- 

 propagating. When a piece of paper is lighted, only a small 

 portion burns initially, but the flame spreads until all the 

 paper is consumed. To get energy from the atom, an atom 

 is required that in disintegiating produces particles that will 

 disintegrate the next atoms they meet. In 1939 some experi- 

 ments showed that such a self-propagating reaction w^as pos- 

 sible for one of the uraniuin isotopes. There are several 

 isotopes of uranium; the coinmon one has an atomic weight 

 of 238. It is radioactive and disintegrates very slowly indeed 

 to form the radium series of elements. Another isotope of 

 uranium has an atomic ^veisrht of 235 and occurs to the ex- 

 tent of 0.7 per cent, or about 14 pounds per ton of uranium. 

 This isotope is disintegrated by the impact of neutrons, but 

 it does not disintegrate by simply emitting one or two par- 

 ticles. The atom actually splits in two, forming two new 

 elements that are first radioactive and then turn into stable 

 elements. This process is known as fission^ and when such a 

 catastrophe happens to an atom, a number of neutrons are 

 emitted. In the case of uranium 235 , as it is called, a neutron 

 starts the reaction, and then it is propagated by the neutrons 

 produced by fission. For this reaction to be propagated 

 through a mass, a certain quantity of 235 is required. Other- 

 wise, so many neutrons escape froin the outside into the air 

 that not enough are available to keep the disintegration go- 

 ing throughout the mass. Also, the 235 must be fairly pure. 



