Silver tone [plated metal] medallion with glass cover and Parkesine mount commemorating the birth Dec 21st 1805 and death Sept 16th 1869 of Thomas Graham. / J.W. Minton Sc. [1869-70?]. diameter 45mm, in mount diameter 58mm. obverse only, Head to L relief profile, signed below. 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 process 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. He had this medal struck from this supposed alloy. An example was presented to Sir John Herschel whom Graham had succeeded as Master of the Mint.