1897-98.] Dr John Shields on Palladium Hydrogen. 171 
resemble that shown in fig. 1. If we operate at a constant tem- 
perature, and start with the solid substance containing no gas 
dissolved in it, then the pressure will also he zero, provided that 
the solid has no vapour pressure of its own at the given tempera- 
ture. If the gas he now gradually dissolved in the solid, the 
pressure will rise continuously, as shown in the diagram, or vice 
versa ; and at any other constant temperature the pressure will 
also vary with variations of concentration, only at a different rate. 
If, on the other hand, a chemical compound is formed on bring- 
ing together the solid substance and the gas, a totally different 
diagram will be obtained, as shown in fig. 2. In this case the first 
introduction of gas produces a small quantity of the compound, 
which has a perfectly definite vapour pressure of its own at the 
given constant temperature. The pressure therefore will imme- 
diately rise to the vapour pressure (dissociation pressure) of the 
compound, and on admitting more gas more of the compound will 
be formed, but the pressure will remain unaltered until the whole 
of the solid substance has been converted into the compound. At 
this point, indicated by the dotted line, the direction of the iso- 
thermal, hitherto parallel with the concentration axis, abruptly 
changes, and if more gas is introduced into the system the pressure 
