Despite the large literature, no satisfactory account has yet been provided of the dynamics of conceptual change involved in the development of the physics of continuous media and fields.¹ The present paper will not consider that rather intractable problem directly, but will restrict itself to a more modest and preliminary consideration. What are the historical stages of conceptual development in the historical emergence of views concerning the propagation of physical action in electrical, magnetic or optical media? Little familiarity with this question is necessary for a realization of the differences between eighteenth-century natural philosophies which regard space as filled with intensive powers, and the elastic aethers of early nineteenth-century optics. Again, both these views are distinguishable from post-Maxwellian concepts of the continuous field. Yet these approaches came together to form part of an important nexus of ideas from which arose interpretations of the physical field as they developed until the end of the nineteenth century. Unless we are able to characterize these similarities and differences, little understanding will be possible of interactions over time between these domains of concepts. As we shall see, the thought of Faraday and Maxwell is somewhat differently related to eighteenth-century doctrines of substantive powers. And Lorentz's work bears a different relationship to the Maxwellian corpus than the ideas of Lord Kelvin. In general, unless we get clearer about descriptive problems of this sort, it will be difficult to give dynamic and historical explanations of the change over time in the refinement and legitimation of concepts.