The phase rule of Willard Gibbs has also played its part in the recent developments in inorganic chemistry. It has laid special stress upon the conditions of equilibrium between the different phases of the same and different substances, and has predicted the existence of unknown substances, many of which have recently been found. The phase rule is a beautiful, short-hand expression of great masses of facts, and it gives us a comprehensive grasp of these facts which without it would be impossible. The application of Faraday' 3 law as the basis of chemical valence is not a new conception, but the importance of this application has only recently become apparent. The importance of ions, which are charged atoms or groups of atoms, and the study of electrochemical phenomena in general have made prominent the fact that the law of Faraday is a fundamental law of chemistry as well as of physical chemistry. If we do not recognize this relation, the term chemical valence is without exact significance and meaning; when based upon the law of Faraday — a law to which thus far no exception is known — valence has an exact physical basis, and this is an impor tant step for the development of chemistry.