diffractometer, which resembled an optical spectrometer. The collimator was
replaced by a lead block pierced by a hole, and the crystal was mounted like a
prism on a rotating table, which could be rotated about a vertical axis. The X-rays were
detected by an ionisation chamber mounted in such a way that it could rotate around
the same vertical axis and connected to a Wilson gold-leaf electroscope. They estimated from their studies on rock salt and pyrite that the wavelength of the X-rays is
0.89 Â 10
À8 Å. They noted the sharpness of the diffraction peaks and surmised that
the waves must occur in trains of great lengths and not in short pulses [21].
These experiments also established the field of X-ray spectroscopy and noted the
relationships between the spectra of successive elements in the periodic table. This
work was followed up by Moseley [41–43] who after working in Rutherford’s
Laboratory in Manchester returned to Oxford and showed using X-ray spectroscopy
that the atomic number, i.e. the number of electrons and protons in a neutral atom,
was the fundamental property of an element and not its atomic weight. This resulted
because he established the relationship between the frequencies of the lines in X-ray
spectra and the element’s atomic number. Moseley showed that the square root of the
frequencies of the corresponding lines in each spectrum increased by a constant
amount when passing from one element to the next using the order of the Mendeleev
periodic table except for nickel and cobalt. The X-ray spectrum of an element is
entirely determined by one integer N which is equal to the charge on the nucleus,
i.e. the atomic number of the element in the periodic table. This analysis led to
Moseley to predict elements which had not been previously characterised, i.e. the
elements technetium (43), promethium (61), hafnium (72) and rhenium (75). The
Braggs’ early work on crystallography recognised that different elements may
diffract the X-rays differently and initially associated this with the atomic weight
of the element, but subsequent results were consistent with Moseley’s
interpretation [21].
In that winter whilst Bragg Sr. was busy designing and supervising the construction of the X-ray spectrometer, Bragg Jr. worked with Pope and Barlow to familiarise himself more fully with how optical crystallographers used Miller indices to
identify crystal faces and the definition of unit cells and space groups [21]. This
knowledge of classical crystallography and the permission to borrow crystals from
the Fitzwilliam Museum in Cambridge enabled the Braggs to solve the structures of
several key compounds and establish the general usefulness of the technique.
The early diffraction studies are summarised in Table 3 and underline how quickly
the subject developed between 1913 and 1933. Bragg noted in his study of ZnS that
the Miller indices of all the spots had indices which were either all even or all odd
and thereby established the usefulness of systematic absences in defining the space
group of a crystal. The use of systematic absences proved to be a very important
feature of early structural determinations. This led to a confirmation that the ZnS
structure was based on an infinite array of tetrahedra and led to a calculation of the
ZnS distance. By 1920 the structures of 50 elements and compounds had been
studied by this new technique although many of the major contributors had been
diverted to assist their governments in war-related activities between 1914 and 1918.
After the war the activity soon picked up, and by 1925 600 structures had been
18
D. M. P. Mingos
replaced by a lead block pierced by a hole, and the crystal was mounted like a
prism on a rotating table, which could be rotated about a vertical axis. The X-rays were
detected by an ionisation chamber mounted in such a way that it could rotate around
the same vertical axis and connected to a Wilson gold-leaf electroscope. They estimated from their studies on rock salt and pyrite that the wavelength of the X-rays is
0.89 Â 10
À8 Å. They noted the sharpness of the diffraction peaks and surmised that
the waves must occur in trains of great lengths and not in short pulses [21].
These experiments also established the field of X-ray spectroscopy and noted the
relationships between the spectra of successive elements in the periodic table. This
work was followed up by Moseley [41–43] who after working in Rutherford’s
Laboratory in Manchester returned to Oxford and showed using X-ray spectroscopy
that the atomic number, i.e. the number of electrons and protons in a neutral atom,
was the fundamental property of an element and not its atomic weight. This resulted
because he established the relationship between the frequencies of the lines in X-ray
spectra and the element’s atomic number. Moseley showed that the square root of the
frequencies of the corresponding lines in each spectrum increased by a constant
amount when passing from one element to the next using the order of the Mendeleev
periodic table except for nickel and cobalt. The X-ray spectrum of an element is
entirely determined by one integer N which is equal to the charge on the nucleus,
i.e. the atomic number of the element in the periodic table. This analysis led to
Moseley to predict elements which had not been previously characterised, i.e. the
elements technetium (43), promethium (61), hafnium (72) and rhenium (75). The
Braggs’ early work on crystallography recognised that different elements may
diffract the X-rays differently and initially associated this with the atomic weight
of the element, but subsequent results were consistent with Moseley’s
interpretation [21].
In that winter whilst Bragg Sr. was busy designing and supervising the construction of the X-ray spectrometer, Bragg Jr. worked with Pope and Barlow to familiarise himself more fully with how optical crystallographers used Miller indices to
identify crystal faces and the definition of unit cells and space groups [21]. This
knowledge of classical crystallography and the permission to borrow crystals from
the Fitzwilliam Museum in Cambridge enabled the Braggs to solve the structures of
several key compounds and establish the general usefulness of the technique.
The early diffraction studies are summarised in Table 3 and underline how quickly
the subject developed between 1913 and 1933. Bragg noted in his study of ZnS that
the Miller indices of all the spots had indices which were either all even or all odd
and thereby established the usefulness of systematic absences in defining the space
group of a crystal. The use of systematic absences proved to be a very important
feature of early structural determinations. This led to a confirmation that the ZnS
structure was based on an infinite array of tetrahedra and led to a calculation of the
ZnS distance. By 1920 the structures of 50 elements and compounds had been
studied by this new technique although many of the major contributors had been
diverted to assist their governments in war-related activities between 1914 and 1918.
After the war the activity soon picked up, and by 1925 600 structures had been
18
D. M. P. Mingos