Pattern half-sovereign or trial piece, inscribed Palladium-Hydrogenium, [Thomas] Graham, 1869. 19mm diameter, in brown case 4x4x1cm. Obverse, head of young Queen Victoria. Reverse, inscription. Inside case inscribed in medal mount: This medal contains 173cc of 900 vols of hydrogen; palladium can absorb up to 900 times its own volume of hydrogen.