Medal commemorating the life of Thomas Graham, 1869.
© Science Museum / Science & Society Picture Library
The medal is inscribed 'Palladium-hydrogenium 1869 Graham'. The chemist Thomas Graham (1805-1869) showed that hydrogen readily penetrated the crystal lattices of certain metals, a proces he called 'occlusion'. Iron, platinum and especially palladium are affected. He supposed that hydrogen gas was the vapour of a very volatile metal hydrogenium which formed an alloy with palladium.