Medal of "Palladium Hydrogenium" struck by and presented to Sir J.F.W. Herschel by Thomas Graham in 1869. Label beneath medal inside case states it contains 147 u or 900 volumes of Hydrogen; 19mm diameter, in leather case 4x4x1.5cm. obverse, inscription Palladium. Hydrogenium. 1869. Graham. ; reverse, head of Queen Victoria, profile to left

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. This example was presented to Sir John Frederick William Herschel whom Graham had succeeded as Master of the Mint.