1897 - 98 .] Dr John Shields on Palladium Hydrogen . 
175 
The point at which the change of curvature takes place, 
however, varies considerably with the nature of the palladium 
employed ; and as it is difficult to know how far the above con- 
clusions are influenced by this fact, and also by the fact that it is 
no easy matter to determine whether equilibrium has or has not 
actually set in, it is very desirable that these conclusions should 
be checked by some independent method. 
It has been deemed necessary to go into some detail with 
regard to the history of this subject, in order to be able to 
explain precisely the object of the following experiments. 
Before quitting this part of the subject, however, it may be 
mentioned that there is other independent evidence against the 
supposed formation of Pd 2 H. Favre ( Compt . rend., lxxvii. 649, 
and lxxviii. 1257) observed that the heat of occlusion of hydrogen 
by palladium remains constant throughout the whole range of 
absorption, and this observation has been confirmed by Mond, 
Bamsay, and Shields ( Phil . Trans., cxci. 105). If a chemical 
compound, say Pd 2 H, were first formed, we should expect *to get a 
certain definite evolution of heat per gram of hydrogen combined 
for the hydrogen first admitted, and then, after sufficient hydrogen 
has been added to form the compound Pd 2 H (about 630 volumes), 
we should expect to find a different value for the heat evolved per 
gram of hydrogen dissolved, or occluded, or absorbed. The fact 
that no differentiation can be observed militates against the view 
that Pd 2 H is formed, although the possible formation of a com- 
pound containing more hydrogen — e.g., Pd 3 H 2 or PdH — is not 
excluded. Again, the same conclusion may be drawn from the 
observation by Dewar {Trans. Roy. Soc. Edin ., xxvii.), and by 
C. G. Knott {Proc. Roy. Soc. Edin., xii. 181), that the increase in 
electrical resistance of a palladium wire is directly proportional to 
the increase in the amount of occluded hydrogen. Measurements 
of the specific gravity and specific heat of hydrogen occluded by 
palladium (Dewar, Trans. Roy. Soc. Edin., xxvii. 167), likewise 
do not lend any support to the view that the definite compound 
Pd 2 H is formed. 
It has been suggested by Dewar {Phil. Mag. (4), xlvii. 334) 
that the composition of fully-charged palladium corresponds to the 
formula Pd 3 H 2 ; and in connection with this view we have the 
