18 Proceedings of the Royal Society 
new truth should dawn upon the mind which has been long and 
intently occupied with a subject than that it should be the fruit 
of a casual and transient consideration. It was by this habit and 
faculty of perseverance that Graham was enabled to do what he 
did ; it was to this that we owe all that he has taught us as to the 
diffusion of gases and liquids, as well as his last and crowning 
discovery as to the nature of hydrogen, of which, perhaps, the full 
effect is not yet fully seen or recognised. 
At an early stage of his inquiries as to hydrogen, he had seen 
that it was isomeric with some of the metals, but his later experi- 
ments went further still towards establishing the metallic character 
of that gas. He showed that certain metals — palladium, platinum, 
and iron — can, under certain circumstances, absorb considerable 
quantities of hydrogen gas. This he termed the “ Occlusion of 
Hydrogen Gas.” Latterly, his investigations were made almost 
exclusively with palladium, which absorbs a much larger propor- 
tion of hydrogen than any other metal. The method he pursued 
was to decompose water by a galvanic battery, the negative elec- 
trode, at which the hydrogen is liberated, being formed of a plate or 
wire of palladium. In this arrangement, when the decomposition 
takes place, oxygen is given off copiously at the positive electrode, 
but no hydrogen, or very little, appears at the negative in the first 
instance, the avidity of the palladium for oxygen requiring that it 
should first be saturated with that substance, after which the 
hydrogen begins to he given off. In this way Graham succeeded 
in charging palladium with a quantity of hydrogen, which, in the 
form of gas, would occupy 900 times the volume of palladium. 
The palladium so charged retains its metallic appearance, and 
differs from pure palladium, very much as a metal containing a 
small quantity of metallic alloy differs from the pure metal. From 
these facts, Graham inferred that hydrogen in its solid state was 
truly metallic, and to this substance, according to the usual ana- 
lysis of chemical nomenclature, the name of hydrogenium was 
given, and a medal of palladium and hydrogenium in the alloyed 
state was struck in honour of the discovery. Another of his recent 
discoveries is said to have been that, while the gas shut up in 
terrestrial iron is carbonic oxide, the gas contained in meteoric 
iron is hydrogen. 
