HYDEOGENIUM. 
237 
leave the metal, and escape into a vacuum, at the temperature of 
its absorption. Pieces of palladium charged with hydrogen 
have been sealed up in exhausted glass tubes. After two 
months the glass has been broken under mercury, and the 
vacuum found perfect, no hydrogen having vaporised in the 
cold, but on the applicatiou of heat 333 volumes of gas were 
evolved from the metal. Another experiment was of a very 
striking character. A hollow palladium cylinder was made the 
negative electrode in an acid fluid, while the closed cavity of 
the cylinder was kept exhausted by means of a Sprengel aspira- 
tor. No hydrogen whatever passed into the vacuous cavity in 
several hours, although the gas was no doubt abundantly 
absorbed by the outer surface of the cylinder, and pervaded the 
metal throughout. 
It appears that when hydrogen is absorbed by the metal 
palladium, the volatility of the gas may be entirely suppressed ; 
and hydrogen may be largely present in metals without exhibiting 
any sensible tension at low temperatures. Occluded hydrogen 
is certainly no longer a gas, whatever may he thought of its 
'physical conditions. 
It has often been maintained on chemical grounds that hy- 
drogen gas — the lightest bod}^ in nature — is the vapour of a highly 
volatile metal. Sir Humphry Davy and others have drawn 
attention, from time to time, to certain conditions which appeared 
to connect hydrogen with the metals, and now the results 
obtained b}^ the Master of the Mint appear to confirm those 
views. Mr. Graham remarks : “ The, idea forces itself upon 
the mind that palladium, with its occluded hydrogen, is simply 
an alloy of this volatile metal, in which the volatility of the 
one element is restrained by its union with the other, and which 
owes its metallic aspect equally to both constituents.” The 
following brief statements of the conditions of palladium — and of 
palladium charged with hydrogen — will elucidate this point. 
It should be stated, in the first place, that palladium in the 
state of thin films, as thrown down from a solution of the chloride 
by a voltaic battery, when heated to 100° in hydrogen, and 
allowed to cool slowly for an hour in the same gas, was found 
to occlude 982*14 volumes of the hydrogen. This is the largest 
absorption of hydrogen which has been observed, and certainly 
it is not a little remarkable to find a dense body, such as the 
metal palladium is, absorbing and retaining nearly one thousand 
times its volume of so light a body as hydrogen is. The density 
of palladium when charged with eight or nine hundred times its 
volume of hydrogen gas is perceptibly lowered. A palladium 
wire before exposure measured 609*144 millims (23*982 inches). 
This wire received a charge of hydrogen amounting to 936 times 
its volume, and increased in length 9*779 millims (or 0*385 
