Rock-salt

Rock-salt, NaCl

As WLB records in his book, The Development of X-ray Analysis(7), published posthumously in 1975: "although the NaCl structure was deduced from Laue's photographs, the first results with the X-ray spectrometer showed at once how much more powerful it was as an analytical tool".

However, the spectrometer data for NaCl in the notebook are tantalizingly sketchy; the index refers only to pages 10 and 53 whereas data for NaCl is given on many more pages, viz pages 4-7, 10, 11 (the pages 'containing the results of previous experiments' including, as noted on p10 'readings taken on paper as the notebook was not available') and pages 30-33, 46-48, 53, 54 and 98. Nor are the θ - 2θ angles for maximum intensity 'starred' (as they clearly are for pyrites) and rough sketch graphs of peak intensities (pages 5, 7, 11) do not correlate with the published data in Table 1 (reproduced in the section on pyrites, FeS2, Figure 7a) except for two angular values (11.8°, p30 and 23.8°, p31) for reflections only from the (100) planes. We must therefore assume that the complete data for NaCl was on paper and is now lost.

These pages in the notebook do however record the problems encountered with the X-ray 'bulbs'. For example on p6 WLB records: "softened bulb" and on p10, "The bulb was blue nearly all over the anticathode spot being very large and a blue line stretching from cathode to anticathode" [a consequence of the bulb containing an excess of residual gas].

There are however, on pages 53 and 98 what must be the first attempts (by WLB) to estimate the absolute values for the homogeneous components (ie Kα) X-ray wavelengths. Given the structure and density of NaCl the unit cell size is deduced and from this, the measured angle of reflection from the (100) planes and a simple application of Bragg's Law, the Kα wavelength for palladium is estimated as 0.576x10-8cm (p 53), corrected to 0.460x10-8cm (p 98). The calculations which WLB pioneered in 1913 are used as undergraduate exercises today!