X. The exact measurement of the specific heat of solid substances at high temperatures: Specific heat, electrical resistance, thermoelectrical behaviour and thermal expansion of some rare‐earth metals

In this paper it is demonstrated that the rare‐earth metals: cerium, lanthanum, neodymium and didymium all show a series of at least four reversible allotropic changes; the different phases are discerned as α‐, β‐, γ‐and δ‐states of these metals. From the study of the specific heat, the thermal expansion, the electrical resistance and the thermoelectrical behaviour in their dependence on the temperature, it becomes obvious that the inner state of all these metals is at each temperature a highly complicated one, it being characterized by the simultaneous co‐existence of several of these modifications. As the transformations between these modifications occur very slowly, in general the instantaneous inner state of these metals is not one of perfect equilibrium and, therefore, depends at each temperature to a high degree on the mode and duration of their preliminary thermal treatment, as well as on the time during which the samples, after treatment, are preserved and again examined. The transition‐temperatures are, in all cases, situated between −170° and about 720° C; the special character of the accompanying retardation‐phenomena is different in each case. More particularly, neodymium manifests most remarkable cyclic effects, a peculiar internal thermo‐electrical force and an abnormal thermal expansion within the limits of a transition‐interval, — all indicating the physical inhomogenity of the metal investigated.