Can you explain why do we write e.g. Fourier's law, Ohm's law, Newton's law of cooling, etc. but Soret effect, Dufour effect instead of Soret's effect, Dufour's effect? What is the principle? Answer The only authority I've been able to find on this particular question is The Oxford Style Manual (2003), which provides this summary: 12.1.6 Eponymic designations Names identified with specific individuals may be treated in several ways. Traditionally a disease, equation, formula, hypothesis, law, principle, rule, syndrome, theorem, or theory named after a person is preceded by the person’s name followed by an apostrophe and an s [examples omitted]; ... An apparatus, coefficient angle, constant, cycle, effect, function, number, phenomenon, process, reagent, synthesis, or field of study named after a person is usually preceded by the name alone or its adjectival form [examples omitted]. Eponymic anatomical or botanical parts may incorporate the name either as a poss...