The model is constructed from balls of wool and steel knitting needles. It is thought to have been made in 1883. Today's molecular models are rather less homely.
Alexander Crum Brown, Professor of Chemistry at Edinburgh 1869-1908, made valuable contributions to stereochemistry, the branch of chemistry concerned with the spatial arrangement of atoms within a molecule. He invented the 'ball and stick' method of representing organic molecules graphically, and the use of short parallel lines to represent a double bond which is still in use today.
The model shows that crystals of rock salt or sodium chloride are small cubes. It is not known what apparatus Crum Brown used, but the model shows that he had realised the ionic nature of the structure, whereas most chemists at the time believed in a molecular structure. The structure was only proved correct by W.L. and W.H. Bragg in 1914 using X-ray crystallography.