In style and content, the physical theory of 1850 shows a marked contrast to that prevalent in 1800. By 1850 the limits and internal cohesion of the science of ‘physics’ were clearly articulated, and the subject had achieved a new and well-defined conceptual content and unity. By 1850 some of the main themes of nineteenth-century physics had been formulated: the unification of physical phenomena within a single explanatory framework, the primacy of mechanical explanation as an explanatory programme, the mathematisation of physical phenomena and the role of mathematical analogy as a guide to the formulation of physical theories, and the enunciation of the principle of energy conservation as a universal, unifying law. The emergence of these broad and unifying themes contrasts with the disunity in physical theory in 1800.

The general disjunction in eighteenth-century physical theory can be illustrated by a contrast between Newton's Philosophiae naturalis principia mathematica [Mathematical principles of natural philosophy] (1687) and his Opticks (1704). In the Principia Newton offered the paradigm of a mathematical science of ‘rational mechanics’, and though he expressed the hope that all physical phenomena could be subsumed under analogous mathematical methods (illustrating his intentions by a mathematical treatment of optical refraction), in the Opticks he based his treatment of the problems of optics and chemistry on an experimental methodology and a speculative theoretical structure, an atomistic physics that became bloated in later editions to include a variety of explanatory agents, forces, active principles, and the ether.