More Change and interaction

Another result that would have delighted old Hegel is the correlation of forces in physics, or the law whereby mechanical motion, i.e. mechanical force (e.g. through friction), is, in given conditions, converted into heat, heat into light, light into chemical affinity, chemical affinity (e.g. in the voltaic pile) into electricity, the latter into magnetism. These transitions may also take place differently, backwards or forwards. An Englishman whose name I can’t recall [Joule] has now shown that these forces pass from one to the other in quite specific quantitative proportions… (Engels to Marx, July 14, 1858.)