Carte de visite photograph by Samuel A Walker & Co. Thomas Graham (1805-1869) showed that hydrogen readily penetrated heated iron, platinum and especially palladium. Palladium absorbs about 650 times its volume of hydrogen which Graham called 'occlusion'. He supposed that hydrogen gas was the vapour of a very volatile metal hydrogenium which formed an alloy with palladium. He had a medal struck from this supposed alloy, an example was presented to Sir John Herschel whom Graham had succeeded as Master of the Mint. Graham was also profesor of chemistry at University College, London.
© Science Museum / Science & Society Picture Library