In a communication made to this Society on December 6, 1897, a tendency towards maximum specific electrolytic conductivity was shown to be a characteristic of many chemical reactions. This tendency is very marked in reactions in which strong acids play a part. Thus, to take a well-known example, chlorine decomposes water under the influence of light with production of hydrogen chloride and oxygen, but as the hydrogen chloride accumulates in the solution, the decomposition of the water is more and more retarded. This photo-chemical reaction is a reversible one, for in the case of highly concentrated solutions the hydrogen chloride is oxidised to water and free chlorine.