CHEMICAL COMBINATION Berthollet, the most distinguished of Lavoisier's colleagues, was one of the group who drew up the new system of chemical nomenclature. He considered he had a decisive proof that chlorine contained oxygen when he had shown that a solution of the gas in water gave off oxygen when exposed to sunlight, and—gradually losing its colour—was reduced to muriatic acid. It remained for Davy to prove that Scheele's gas contained no oxygen, and that chlorine—the non-committal name suggested by him—was one of a family of elements having acid-forming hydrides.
Berthollet's chief claim to fame rests on his statement of the Law of Mass Action (see CHEMICAL ACTION), that the amount of a compound formed by an element A with B or with C depends, not only on the relative affinity of A for B and for C, respectively, but also on the quantities of B and C which are present and in a state to interact with A. This law has been of great importance in theoretical chemistry as well as in manufacturing operations.
But Berthollet carried his doctrine further: he considered that the constancy of composition shown by most known compounds depended on certain conditions prevailing at their preparation. When two elements were combining, a particular composition might correspond with insolubility or volatility, and so this com pound might be separated out. Under other conditions the two elements might unite to form compounds in proportions varying between certain limits depending on the active masses of the elements. Berthollet instanced the oxides of lead and of other metals as showing such varying composition. This conception was opposed by Proust, and a long controversy ensued in which it was proved that the "varying" oxides were mixtures of two or more definite oxides, while it was still maintained that liquids such as alcohol and sulphuric acid combined with water in all proportions.