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

358 THE TIME OF THE GREAT DEPRESSION (1931-40)
occupying the same space in the Periodic Table of Elements even though they
possessed different atomic weights. He coined the word "isotope" to de-
scribe chemically identical substances with different atomic weights.
The 1920's saw the development of the mass spectroscope, an electro-
magnetic device that sorted out atoms, both normal and isotopic, according to
their masses, and projected them as sharp clear lines in a spectrum. Analysis
of the true weights of the atoms of various elements thus became possible,
and with this instrument, F. W. Aston of the Cavendish Laboratory at Cam-
bridge showed that not only radioactive elements, but almost all elements,
have isotopes.171
One element, hydrogen, gave particular trouble. Precise measure-
ment of its atomic weight indicated that it had a heavy isotope, but apparently
in so small concentration that Aston could find no trace of it on the spectro-
scope. At this point the Bureau of Standards became actively associated with
this field of atomic research.
In the summer of 1931, Harold C. Urey, associate professor of chem-
istry at Columbia, then lecturing at the Johns Hopkins University, became
convinced .that an isotope of hydrogen of mass 2, though unknown, could
be found.172 In conversation with Fred L. Mohier of the atomic physics sec-
tion of the Bureau, Urey told him that in his studies of the hydrogen spectrum
he had found a satellite line next to the hydrogen alpha line that he thought
might be heavy hydrogen. Urey sought a way to enrich the suspected isotope,
and wondered whether liquid hydrogen might not make better definition
possible. Mohler suggested the Bureau's cryogenic laboratory, where Brick-
wedde was studying ortho- and para-hydrogen. There, successive low-
temperature distillations of liquid hydrogen resulted in a concentration whose
spectrum left no doubt of the existence of the isotope.'73
Urey had earlier suggested the possibility of separation of the isotope
by electrolysis, but the procedure had been tried and given up as unpromising.
Acting on a suggestion of Dr. Washburn, chief of the Bureau chemistry divi.
sion, Edgar R. Smith on December 9, 1931, began an experiment in the iso-
topic fractionation of water by repeated hydrolysis of solutions of caustic
potash. When 98 percent of the water had been decomposed in this manner,
the density of the hydrogen in the residual water proved measurably higher
"In the early nineteen-thirties, many atomic masses were very accurately measured and
the energy of products of many nuclear reactions became known * * * [making) pos-
sible the full quantitative verification of Einstein's 1905 prediction that mass and energy
are equivalent." E. U. Condon, "Physics," pp. 145—146.
See Urey, "The natural system of atomic nuclei," J. Am. Cml. Soc. 53, 2872 (1931).
NBS TNB No. 179 (March 1932), p. 23; Urey, Brickwedde, and Murphy, "A hydrogen
isotope of mass 2 and its concentration," Phys. Rev. 40, 1 (April 1932) ; interview
with Dr. Mohler, Oct. 1, 1963.