1897 - 98 .] Dr John Shields on Palladium Hydrogen. 173 
241, etc.), in the absorption of ammonia by silver chloride (Horst- 
mann, Ber. deutsch. Ghem. Gesell., ix. 749; Isambert, Compt. 
rend., vols. lxvi. and ixx.), etc. 
This criterion was first applied to hydrogenised palladium in 
1874 by Troost and Hautefeuille {Ann. cliim. plugs. (5), ii. 279), 
who discovered that the pressure-concentration curve was hori- 
zontal or nearly horizontal— i.e., there existed a constant or nearly 
constant dissociation pressure over a certain range, from which 
they concluded that the definite compound Pd 2 II was formed, and 
that the excess of hydrogen over and above that which was 
required for the formation of this compound was simply occluded 
or dissolved in the ordinary way. 
This conclusion, however, has been called in question by Hoitsema 
( Zeits.f . yhysikal. Chem ., xvii. 1), who, from more extended and 
critical investigations by himself and Rooseboom, concludes that 
the measurements of the dissociation pressures furnish no evidence 
in favour of the existence of Pd 2 II, or of any definite chemical 
compound. The only interpretation which he can give of the 
results obtained, and this is given under due reserve, is that two 
immiscible solid solutions are formed. 
As the results obtained by the Dutch investigators are extremely 
interesting, some of them have been reproduced graphically in 
fig. 3, which shows how the pressure of palladium hydrogen varies 
at different temperatures with the concentration of the hydrogen. 
In the diagram the, ordinates represent the pressure in centimetres 
of mercury, whilst the abscissae indicate the number of atoms of 
hydrogen associated with one atom of palladium. 
Prom a careful study of the diagram, Hoitsema insists on three 
points, — first, the middle nearly horizontal part of the curves is 
not absolutely horizontal as it ought to be, and as it is in the case 
of other diagrams (Andreae, loc. cit.) ; second, the change of curva- 
ture at the right-hand end of the nearly horizontal part is not 
abrupt but very gradual ; third, the concentration of the hydrogen 
at the point corresponding to this change of curvature does not 
remain constant but varies for each isothermal. Hone of the 
criteria, therefore, which are characteristic of a chemical com- 
pound are fulfilled in the above diagram. With reference to the 
supposed formation of Pd 2 H it was purely accidental, that in 
