1897-98.] Dr John Shields on Palladium Hydrogen. 
169 
On the Nature of “Palladium Hydrogen.” By John 
Shields, D.Sc., Ph.D., Davy-Faraday Laboratory, Boyal 
Institution, London. 
(Read March. 7, 1898.) 
Ever since the remarkable discovery by Graham in 1866 that 
palladium possesses the power of occluding hydrogen gas in large 
quantity, various views have from time to time been put forward 
to explain the true nature of the phenomenon. Whilst some 
observers regard hydrogenised palladium as an alloy or solid solu- 
tion, others again consider it to be, or at least to contain, a definite 
chemical compound or hydride. Those who hold the latter 
opinion, however, are not agreed as to which compound or hydride 
is formed, as is proved by the fact that different formulae have 
been ascribed to it. Graham himself recognised The possibility 
that a definite chemical compound might be formed, for he says 
( Researches , 287) that in fully-charged palladium there exists one 
equivalent of palladium to 0772 equivalent of hydrogen, or an 
approximation to single equivalents which would be represented 
by the formula PdH. His opinion was, nevertheless, opposed to 
the idea of such a definite chemical combination, one of his chief 
objections being that no visible change is occasioned to metallic 
palladium by its association with hydrogen. He regarded the pro- 
duct simply as an alloy of the volatile metal luydrogenium, 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. 
Considerations of a purely chemical character have up to the 
present time proved insufficient to decide which of these views is 
correct. The aid of physical methods has also been invoked, but 
the results achieved are not quite so satisfactory as might be 
desired. On certain points, however, evidence of a fairly conclu- 
sive character has been produced, and before describing some 
YOL. XXII. 14/9/98 M 
