The nature of solutions has also been developed by the studies of Ostwald and Van’t Hoff, and as a result of these studies, a flood of light has been thrown upon the constitution of compound bodies. In the development of physical chemistry, attention should be directed to the help afforded by Newlands (1864) and Mendelejeff (1869) and others, showing that the elements form groups which tend to recur with a periodicity which is sufficiently definite to enable the investigator to foretell to some extent the properties of the elements which have never yet been discovered, and whose existence is necessary in order to fill up the gaps in existing groups. By this method the existence, atomic weight and properties of scandium, gallium, and germanium were foretold years before their discovery. Such actual realization of a scientific-prophetic method is one of the strongest indications of the basis of fact upon which it rests. Although a rigid application of the principles of the periodic law is not possible, yet its discovery and elaboration mark one of the great forward steps of chemical philosophy. If we regard any material system by itself, i.e., independently of any other system or influence by which it may be surrounded, we recognize it as consisting of essentially two things,—matter and energy. A precise definition of either matter or energy is difficult, if not impossible; but what is connoted by these names is sufficiently well understood by their well-known properties. Both energy and matter are essential to each and every system. They are coexistent. In the light of human experience, we cannot conceive of one existing without the other; and in the study of any material system, consideration of one of these components without the other can only be regarded as incomplete. But, for the sake of convenience, this has been the practice, and, generally speaking, chemists have concerned themselves with matter changes of equilibria, while physicists have more especially directed their attention to energy equilibria. The object of the physical chemist is to follow equilibria changes in given systems, having due regard for both the matter and energy involved. Berthollet may be regarded as the first true physical chemist, on account of his classical views on mass action. Largely because the time was not ripe for it, his views were not generally adopted.