PUBLISHED M A Y 5, 1926After Graham reported the great absorptive capacity of palladium for hydrogen, the relationship between the two elements became the subject of much experimental study. Besides the interest of the possible compound Pd2H, the relationship may have bearing on the great catalytic activity of palladium in reduction.They brought more evidence forward than they considered necessary. When the phase rule was announced a few years later, it could be seen that their evidence, though almost complete, was not quite so. A number of workers have since tried to furnish a complete proof, but without success.Pressure-composition diagrams, obtained by Roozeboom and by Hoitsema for the system Pd-H, were presented by the latterW3 Hoitsema concluded that the system was isothermally univariant throughout and that, therefore, the compound PdzH was not formed. In coming to this conclusion he was not in a position to explain the possibility of observations such as those of "roost and Hautefeuille. Nevertheless, his conclusion as to the nonexistence of the hydride has been generally accepted; and although many writers show a feeling that all is not right, we have learned of only one writer4 (up to 1925) who regards the existence of PdzH as an open question.%everts5 obtained curves very similar to those of Hoitsema. The curves show no nonvariant portions and the "breaks" are not sharp.Holt, Edgar and Firth6 postulate the presence of an amorphous and a crystalline variety of palladium but have hardly demonstrated the presence of these two distinct modifications. The work of A. E. Oxley' on the determination of the magnetic properties of palladium that has been * This paper is an abstract of a thesis presented by F. P. Hall in partial fulfilment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology. We wish to acknowledge the assistance of Mr. Emanuel Tarplin during part of the experimentation.In 1874 "roost and Hautefeuille2 reported the hydride Pd2H. t-Troost and Hautefeuille, Comfit. rend., 78, 686 (1874); Ann. chim. phys., 151 2, 273 (1874).