402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
The major interest of physical science in the first dozen years of the 
century was in the attempt to explain natural phenomena by the 
behavior of electrons under the influence of electric forces. Such 
theories were very successful for some phenomena, and had some 
very important practical applications, namely, our entire modern 
electronics industry. But the electron alone was far from adequate 
to account for the universe. 
Then Sir Ernest Rutherford proved that each atom has a heavy 
nucleus of positive electricity surrounded by electrons. Moseley in 
England proved by X-rays that these atomic nuclei are characterized 
by simple numbers: 1 for hydrogen, 2 for helium, 3 for lithium, and 
so on up to 92 for uranium—and these numbers were soon identified 
with the electric charge of the nucleus or the number of electrons 
outside it in the atom. Thus quantitative meaning was given to the 
periodic table of the chemists. Next, Bohr in Denmark and Sommer- 
feld in Germany applied the quantum theory to the Rutherford- 
Moseley atom and found the basis for explaining the spectra of light 
and X-rays. Henceforth spectroscopy became the most powerful tool 
for further atomic structure research, and such research became a 
major preoccupation of physicists in the 1920’s. 
But all during this time other scientists were experimenting with 
radioactivity, an interesting and puzzling subject whose only practical 
uses had been for making watch dials luminous, and treating with 
moderate success certain types of cancer. But when Rutherford in 
1920 succeeded in transmuting one chemical element into another by 
bombarding it with fast particles from a radium source, and thus made 
real the ancient dream of the alchemists, a new era in science opened 
up. It opened slowly at first, and it was not until 1931 that such a 
transmutation was effected by use of a high-voltage machine. This 
was done by two pupils of Rutherford in Cambridge University. In 
that same year Ernest Lawrence at the University of California 
invented the cyclotron, which has proved the most productive of all 
atom-smashing machines to date. Also in the same year, Chadwick 
in England discovered another very important subatomic particle, the 
neutron. And still in that same year Fermi in Italy showed that 
neutrons are extremely potent in producing atomic transmutations in 
the atoms which they strike. 
The quick result of the atomic nuclear research stimulated by these 
discoveries was the new discovery, or production in the laboratory, of 
more than twice as many new species of atoms as had been previously 
known to exist. Furthermore, whereas it was formerly thought that 
only a very few of the heaviest types of atoms were radioactive, it is 
now possible in these atom-smashing machines to produce at least one 
radioactive modification, or isotope, of every kind of chemical atom, 
and several radioactive modifications in many cases. 
