6 - Beyond molecular vortices

Summary

The theory of molecular vortices had constituted the focus of Maxwell's research program in electricity and magnetism in the late 1850s and early 1860s, and his two major innovations of that period – the introduction of the displacement current and the treatment of electromagnetism and optics within a single theoretical framework – grew out of the theory of molecular vortices and reflected that context in their initial formulations. In the course of Maxwell's elaboration of the molecular-vortex model, however, problems had accumulated, to the point that he had serious reservations concerning certain parts of the model. In addition, Maxwell's research program in the theory of heat and gases was, in the years around 1860 and thereafter, developing in such a way as to undermine support for the theory of molecular vortices in that area, which had been its original stronghold. Finally, and relatedly, Maxwell's general views on the use of mechanical models in science were developing in a new direction that involved less emphasis on specific and concrete models. All of these factors converged in encouraging Maxwell to begin a measured retreat from the molecular-vortex model.

As part of this general retreat from the model, Maxwell took steps to free his signal innovations in electromagnetic theory from their original matrix in the theory of molecular vortices. The modification of Ampère's law and, more significantly, the incorporation of optics into electromagnetic theory defined new research programs, based on those innovations.