Salt. A model of the atomic structure of crystalline common salt (sodium chloride). This shows the cubic structure of alternating sodium and chlorine atoms. The model was used as a teaching aid by Sir William H Bragg in the 1920s and 1930s. Bragg was appointed Fullerian Professor of Chemistry at the Royal Institution in 1923 and used such models to teach the theory of x-ray diffraction. The model is preserved at the Royal Institution.