THE world of chemical phenomena is so immensely wide, and the phenomena themselves are so complicated, that the founders of modern chemistry were compelled to limit the area of their investigations, and sharply to separate their own domain from those of the two sister-sciences, physics and mechanics, leaving it to the future to find out the bonds which might unite all three branches into one harmonious whole. They and their followers elaborated their own methods of investigation; they discovered their own chemical laws and worked out their own hypotheses and theories; and, with the aid of these methods, laws, and hypotheses, they created a science which not only interprets, discovers, and predicts the phenomena it deals with, but already has brought us within a measurable distance of a general theory of the structure of matter altogether.
In proportion as chemical research went deeper into the study of the wonderful movements and interactions of molecules and atoms, the intimate connection which exists between chemistry, physics, and mechanics became more and more apparent. The physical and the chemical properties of matter proved to be so closely interdependent that they could be explained no longer with the aid of chemical theories alone; the very fundamental laws of chemistry appeared to be but so many expressions of physical facts; and chemistry stands now in such a position that no further advance in its theoretical part is possible, unless it enters the border-land which separates it from physics, recognizes the unity of chemical and physical forces, and, availing itself of the progress recently made in molecular mechanics, boldly attacks the great problem of a physical—that is, a mechanical—interpretation of chemical facts. This is the work which now engrosses the attention of most chemists.
The points of contact between physics and chemistry are very numerous, and the work is being carried on in several directions at once. The discovery by Mendeléeff of the so-called "periodical law of elements" has called into life numerous researches, some of which accumulate correct numerical data to express the dependence between the physical properties of various bodies and their chemical constitution; while others endeavor to interpret this very periodicity in the properties of the elements under the assumption of their compound nature. On the other side, the recent development of the mechanical theory of heat, and the