the first law of photochemistry, which states that chemical reaction in a substance can be brought about by the absorbed part of the incident light only. First stated by K. Grotthus in 1818 on the basis of theoretical considerations. The position was supported experimentally by J. Herschel in 1842 and J. Draper in 1843. (It is also called the Grotthus-Draper law.) The Grotthus law was very important as the precursor of the law of photochemical equivalence, discovered by A. Einstein in 1912–13.