 Long after his death, in the first decades of the 20th century, Berthollet was partially vindicated with the careful characterization of a class of substances that we now call Berthollides. These are compounds whose proportions of elements do not stand in simple relations to one another. Their elemental proportions are not fixed, but vary with temperature and pressure. For example, the mineral wüstite, or ferrous oxide, has an approximate compositional formula of FeO, but typically has somewhat less iron than oxygen.