 These two equations on the four infinitesimal increments normally constrain them to a two-dimensional linear subspace space of possible infinitesimal state changes, that depends on the material and on the state.The constant-volume and constant-pressure changes are only two particular directions in this space.This analysis also holds no matter how the energy increment formula_59 is injected into the sample (by heat conduction, irradiation, electromagnetic induction, radioactive decay, etc.