Rosalind Franklin used helium to determine coal density during her studies of coal porosity. Helium was especially useful because its small, chemically inert atoms could penetrate the fine pore spaces in coal.
Franklin joined the British Coal Utilisation Research Association in 1942, during the Second World War. She investigated how coal’s microscopic structure affected its density, permeability and behaviour when heated. Comparing helium measurements with liquids and other materials helped distinguish true density from apparent density.
Her work showed that coal was not simply a uniform solid. Its pores varied in size and acted rather like a molecular sieve: small particles could enter spaces that larger molecules could not. Heating coal changed those structures and affected which substances could pass through.
Nitrogen, hydrogen and argon can also be used in modern density or adsorption experiments, which may cause confusion. Franklin’s classic coal-density work, however, is specifically associated with helium. The research formed the basis of her 1945 Cambridge PhD.