156 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946: 
The answer is that these reactions can be produced in the laboratory 
only on an infinitesimal scale by the expenditure of far more energy 
than they release. The nuclear reactions observed before 1939 are 
fundamentally impractical as commercial sources of power because 
they are not self-propagating. <A fire once started will continue to 
burn as long as fuel is available. But in the proton-lithium reaction 
just mentioned, the few alpha particles formed do not lead to other 
reactions with additional release of energy. Subatomic energy, if 
it 1s to be harnessed efficiently, must start a chain reaction that will 
proceed to release energy of its own accord. 
Two decades after Rutherford launched physicists upon an orgy 
of atom splitting, Hahn and Strassemann?° reported a new type of 
nuclear reaction which has become of transcendental importance. 
‘Following up experiments by Fermi, Irene Curie, and others, they had 
bombarded uranium with neutrons and obtained some radioactive 
substances whose chemical properties resembled barium. This led 
them to think that these new substances were probably isotopes of 
radium, which is chemically similar to barium. Further analysis, 
however, failed to distinguish these products from barium, and at 
last they were forced to conclude that the new substances actually were 
isotopes of barium. All previous experiments on nuclear disinte- 
gration had succeeded only in transforming the reacting elements into 
others of slightly different atomic weight, such as nitrogen to oxygen, 
beryllium to carbon, ete. But in this experiment, neutron bombard- 
ment of uranium of atomic number 92 at the end of the periodic table 
had resulted in the appearance of an element of atomic number 56 
near the middle of the table. 
The experimental result also conflicted with theoretical ideas con- 
cerning the nucleus. Previously it had been supposed that the rapid 
emission of large numbers of charged particles from the nucleus was 
precluded by the “Coulomb barrier,” or potential field of force sur- 
rounding the nucleus. This was pictured as a steep, craterlike barri- 
cade which prevented exterior particles from reaching the nucleus, as 
well as preventing the nuclear particles from escaping. Using a new 
nuclear model proposed by Bohr™ 2 years before, Lise Meitner and 
O. R. Frisch ” showed that a rather simple explanation was possible. 
In Bohr’s theory, the particles composing the nucleus of a heavy atom 
can be considered as behaving somewhat like the molecules in a drop 
of water. Collectively, the molecules cling tightly together, but occa- 
sionally one near the surface, through a series of favorable collisions, 
will gain sufficient speed to fly off or “evaporate.” In the nuclear case, 
10 Naturwiss., vol. 26, p. 756, 1938. 
1 Nature, vol. 137, pp. 344, 351, 1986. 
12 Tbid., vol. 1438, p. 239, 1939. 
