643 
HYDROGENIUM; THE RELATION OF HYDROGEN TO 
PALLADIUM. 
It has been shown by Mr. Graham, the Master of the Mint,* that palladium, 
platinum, and some other metals, are capable, at an elevated temperature, of 
absorbing considerable quantities of hydrogen, which is afterwards retained 
when the metal has cooled, but may be separated by heating the metal in 
vacuo. 
Palladium appears to possess the most remarkable property of thus absorbing 
hydrogen. At a temperature a little below that of boiling water, Graham 
found that one volume of palladium absorbed and retained 643 volumes of hy¬ 
drogen. 
Platinum possesses the same property, but in a lower degree. Speaking of 
this metal, Mr. Graham says, “ It appears necessary to recognize in platinum a 
new property, a power to absorb hydrogen at a red heat, and to retain that gas 
at a temperature under redness for an indefinite time. It may be allowable to 
speak of this as a power to occlude (to shut up) hydrogen, and the result as the 
occlusion of hydrogen by platinum.” 
The term occlusion has thus been introduced to express the property in metals 
of absorbing gases. 
In a paper communicated to the Royal Society on the 22nd May, 1868, “ On 
the Occlusion of Hydrogen Gas by Metals,” Mr. Graham described a new me¬ 
thod of causing the absorption of hydrogen by metals, which consisted in mak¬ 
ing the metal the negative electrode of a voltaic arrangement, including six 
cells of a Bunsen’s battery. When palladium was thus used, the evolution of 
oxygen gas at the positive electrode continuing copious, the effervescence at the 
negative electrode was entirely suspended for the first twenty seconds, in con¬ 
sequence of the hydrogen being occluded by the palladium. The final absorp¬ 
tion amounted to 200 volumes. 
Mr. Graham says, u It is worthy of remark that, although the hydrogen en¬ 
ters the palladium and no doubt pervades the whole mass of the metal in such 
circumstances, the gas exhibits no disposition to leave the metal and escape into 
a vacuum at the temperature of its absorption. Thus, a thin plate of palladium, 
charged with hydrogen in the manner described, was washed, dried by a cloth, 
and then sealed up in an exhausted glass tube. On breaking the tube under 
mercury after two months, the vacuum was found perfect. No hydrogen had 
vaporized in the cold (about 12° Cent.) ; but on the application afterwards of a 
heat of 100° Cent, and upwards, 333 volumes of gas were evolved from the 
metal.” 
“It appears, then, that when hydrogen is absorbed by palladium, the vola¬ 
tility of the gas may be entirely suppressed ; and hydrogen may be largely pre¬ 
sent in metals without exhibiting any sensible tension at low temperatures. 
Occluded hydrogen is certainly no longer a gas, whatever may be thought of 
its physical condition. The same conclusion was indicated by another series of 
experiments, in which it was found that, to be occluded by palladium, and even 
by iron, hydrogen does not require to be applied under much pressure, but, on 
the contrary, when highly raritied is still freely absorbed by these metals. 
“The occluded hydrogen is readily extracted from palladium by reversing 
the position of the latter in the decomposing cell of the battery, so as to cause 
oxygen to be evolved on the surface of the metal. The hydrogen is then drawn 
out as rapidly as it had previously entered the palladium, and the metal is ex¬ 
hausted in a complete manner by such treatment. When palladium charged 
* Philosophical Transactions, 1866. 
