Apparatus used by the chemist Thomas Graham, c 1860s.
© Science Museum / Science & Society Picture Library
Thomas Graham (1805-1869) was primarily interested in the physical factors relevant to chemical reactions. His name is remembered in Graham's Law which governs the rate at which gases of different densities will diffuse through small apertures. To the left is a Graham osmometer, where a semi-permeable membrane is fitted over the end of the bell jar. The glas bowl uses the dialysis technique which in this case separated a solution of salt and glue.