These theoretical advances eventually resulted in the superseding of classical electromagnetism by quantum electrodynamics. These include classical mechanics, quantum mechanics, thermodynamics and statistical mechanics, electromagnetism, and special relativity. Magnetism is one aspect of the combined phenomenon of electromagnetism. This is analogous to mass currents in hydrodynamics and electric currents in electromagnetism. In particular, they include the laws of quantum mechanics, electromagnetism and statistical mechanics. Important forces include the gravitational force and the Lorentz force for electromagnetism. Hence they interact with other fermions via electromagnetism and the weak interaction. Aside from the properties described in articles about electromagnetism, charge is a relativistic invariant. This article summarizes equations in the theory of electromagnetism. Some astrologers have proposed conventional causal agents such as electromagnetism and gravity. Theoretical investigation in classical electromagnetism led to the discovery of wave propagation. Grassmann also wrote on crystallography, electromagnetism, and mechanics. Faraday suffered a nervous breakdown in 1839 but eventually returned to his investigations into electromagnetism. Parity symmetry appears to be valid for all reactions involving electromagnetism and strong interactions. Thomson published a number of papers addressing both mathematical and experimental issues of electromagnetism.