Berthollet, Claude Louis

Born Dec. 9, 1748, in Talloires, Savoy; died Nov. 6,1822, in Paris. French chemist; founder of the study of chemical equilibrium. Member of the Paris Academy of Sciences (1785). Professor of chemistry at the Normal and Polytechnic schools in Paris (1794).

Having received the degree of doctor of medicine in Turin in 1770, Berthollet began to work in pharmacies; in 1772 he moved to Paris. In 1785 he was the first chemist to associate himself with the antiphlogiston views of A. Lavoisier. Together with Lavoisier and other scientists, Berthollet participated in the development of a new chemical nomenclature (1786–87) and in the founding (1789) of the journal Annales de chimie, which still exists.

During the Great French Revolution, Berthollet took an active part in organizing the production of saltpeter, steel, and other materials needed for the republic’s defense. In 1798–99 he participated in Bonaparte’s Egyptian expedition. In 1807, Berthollet founded a scientific society whose members included P. S. Laplace, A. Humboldt, J. B. Biot, D. F. Arago, and J. L. Gay-Lussac.

Particularly significant in Berthollet’s experimental work was his procedure for bleaching linen, wax, and paper with chlorine (1785) and the discovery, associated with this, of hypochlorite, chlorate, and Berthollet salt. Berthollet established that the direction of chemical reactions is determined by mass and the properties of the reacting substances and the conditions of the reaction. Proceeding from a view of the chemical reaction as a continuous and reversible process, Berthollet felt that the composition of the compounds formed during the process should change continuously, that is, should be variable. The French chemist J. L. Proust opposed this conclusion; he showed that the substances cited by Berthollet as examples of compounds of variable composition were in actuality mixtures and not chemical units. The controversy between Berthollet and Proust (1801–08) ended with the establishment of the law of definite proportions, which strengthened the position of the atomic theory in chemistry and concentrated the efforts of scientists on obtaining and investigating compounds of constant composition. At the start of the 20th century, N. S. Kurnakov discovered the existence of the chemically individual substances of variable composition foreseen by Berthollet. He named them berthollides in memory of Berthollet. This discovery resolved the contradiction between the seemingly incompatible views of Berthollet and Proust.