In the nineteenth century the term ‘physics’ acquired new and significant connotations. Although the term was still occasionally used in the traditional sense to refer to natural science in general, by the early nineteenth century ‘physics’ was being used in the modern and more specialised sense to denote the study of mechanics, electricity, and optics, employing a mathematical and experimental methodology. In the article entitled ‘Physical Sciences’ in the ninth edition of the Encyclopaedia Britannica in the 1870s, James Clerk Maxwell identified the scope of physics with the programme of mechanical explanation, first enunciated in the seventeenth-century ‘mechanisation of the world picture’, which sought to explain physical phenomena in terms of the structure and laws of motion of a mechanical system. In a critical exposition of current physical theory, The concepts and theories of modern physics (1881), Johann Bernhard Stallo gave an informative and more detailed definition of the theoretical structure of physics as conceived by contemporary theorists:
The science of physics, in addition to the general laws of dynamics and their application to the interaction of solid, liquid and gaseous bodies, embraces the theory of those agents which were formerly designated as imponderables – light, heat, electricity and magnetism, etc.; and all these are now treated as forms of motion, as different manifestations of the same fundamental energy.
In the nineteenth century the science of physics came to be defined in terms of the unifying role of the concept of energy and the programme of mechanical explanation.