History of Science and Engineering Annotation << Back Bertollides of Academician Kurnakov Nipan G.D., Smirnova M.N. The history of the emergence of physicochemical analysis as an independent scientific discipline at the beginning of the 20th century is unusual and instructive. Academician Kurnakov, the creator of a new section of chemistry, proved, based on experimental material, that real crystals of compounds have a variable composition, are limited solid solutions, and, contrary to the postulates of inorganic chemistry, the applicability of the laws of constancy of composition and multiple ratios is limited. Berthollet's doctrine of mobile equilibrium, rejected by inorganic chemistry, turned out to be an effective tool in describing phase transformations, and the use of geometric images proposed by Kurnakov to describe stable and metastable states of multicomponent systems facilitated the work of materials scientists and ensured the rapid development of physicochemical analysis. Eliminating contradictions between confl icting sections of chemistry, Kurnakov introduced the concepts of "daltonide" (the stoichiometric composition of the compound falls within the homogeneity region of a non-stoichiometric crystal) and "berthollide" (the stoichiometric composition is outside the homogeneity region) for limited solid solutions. The article presents the results of the author's physicochemical studies of limited solid solutions within the pressuretemperature-chemical composition space of the late 20th century – early 21st century, with a consistent transition from twocomponent systems to three- and four-component systems. Keywords: physicochemical analysis, limited solid solutions, Berthollet's teaching, chemistry, history of chemistry. DOI: 10.25791/intstg.5.2025.1548 Pp. 26-35.