"Anyone who had studied the vicissitudes of atomic theory during the period between 1810 and 1860 recognizes the tremendous problems which faced chemists of that day in connection with atomic weights, equivalent weights, reliable formulas, and matters of that sort. The Law of Definite Proportions was a useful concept in helping bring order out of chaos. ...Had chemists had to face the fact of variable composition in some of their common compounds it is doubtful if atomic theory might have been established as soon as it was. It is in solid state chemistry that the Law of Definite Proportions has been found wanting. Not only in the case of metallic compounds are peculiar atomic ratios of the component elements to be found, but even in such solids as metallic oxides and sulfides. Ferrous oxide (FeO) presents a particularly fine example... Although the compound is frequently mentioned in freshman chemistry courses to illustrate the Law of Definite Proportions... on accurate analysis... the ratio is somewhat between the range of 0.84 to 0.95 atoms of iron per atom of oxygen. ...The existence of a considerable number of such compounds has led to the proposal that compounds be classified as Berthollides and Daltonides; the term Berthollide referring to such compounds as cuprous sulfide with a somewhat variable domposition, and Daltonide referring to those with precisely fixed atomic ratios."