Measures for Progress: A History of the National Bureau of Standards

360 THE TIME OF THE GREAT DEPRESSION (1931-40)
part in the final process of nuclear disintegration, the deuteron was not to be
the trigger of atomic power as the chemists hoped. This role was reserved
for the neutron.
Enrico Fermi, then in England, reasoned that neutrons, lacking
charge, should be highly effective in penetrating nuclei, especially those of
high atomic number, with consequent release of energy. He selected
uranium, at No. 92 the last of the naturally occurring elements, with atomic
weight 238. But his bombardment in 1934 of uranium by neutrons slowed
down by the use of deuterium proved inconclusive. He obtained only a
"confusion" of radioactive substances, two of which, however, proved to
have atomic numbers larger than 92. In Germany in late 1938, Otto Hahn
and Fritz Strassmann at the Kaiser Wilhelm Institute for Chemistry per-
formed the same experiment and obtained a large variety of radioactive
isotopes of chemical elements having half the atomic weight of the original
uranium. Announcement of the significance of these two findings was near.
Lise Meitner in Sweden, a refugee physicist from Germany, and her
nephew Otto H. Frisch in Denmark, informed of the work of their former
colleagues in Berlin and pursuing Fermi's line of investigation, hit on the
answer. Meitner and Frisch conjectured that the uranium nucleus, with low
stability of form, had divided into two nuclei of roughly equal size, releasing
in the process enormous quantities of energy—the "confusion" Fermi had
observed. They estimated the total energy resulting from that splitting of
the uranium atom as about 200 MeV (200 million electron volts) •179 Their
letter explaining the Hahn and Strassmann observations and Frisch's experi-
mental verification, "Disintegration of uranium by neutrons: a new type of
nuclear reaction," appeared in Nature magazine on February 11, 1939.180
With full knowledge of Frisch's experiments, Niels Bohr arrived in
this country the month before the appearance of the paper, and while visit-
ing at Princeton told Einstein, in residence there, and Eugene P. Wigner,
Princeton professor of theoretical physics, of its import. He also saw Prof.
George B. Pegram at Columhia and Fermi, who had come to work in Pegram's
laboratory. He impressed on them the significance of these experiments,
told them of the work of Hahn and Strassmann, and something they did not
'79The sum of the mass of the 2 fission fragments, totaling less than the mass of the
original uranium nucleus, suggested that the matter that had disappeared had been
transformed into energy. Although the matter transformed was small, Einstein's formula
E—mc' indicated the enormity of the energy released, considering that the mass must
be multiplied by the square of the speed of light, which is 185,000 miles per second.
James P. Baxter, Scientists Against Time (Boston; Little, Brown, 1946), p. 420.
180 Nature, 143, 239. Niels Bohr's account of the verification, confirmed to him in a
telegram from Frisch, appeared in this country in Phys. Rev. 55, 418 (Feb. 15, 1939).