Full Idea
Helmholtz provided the requisite mathematical formulation linking heat, light, electricity and magnetism, by treating these phenomena as different manifestations of 'energy'.
Gist of Idea
Helmholtz used 'energy' to mathematically link heat, light, electricity and magnetism
Source
report of Hermann von Helmholtz (On the Conservation of Force [1847]) by Peter Watson - Convergence 01 'Human'
Book Reference
Watson,Peter: 'Convergence' [Simon and Schuster 2016], p.28
A Reaction
I'm increasingly struck by the neglect by philosophers of nature of these amazing developments in 19th century physics, because they prefer the excitement of the latest nuclear physics. There is more philosophical interest in the earlier stages.