This year marks the 90th anniversary of the publication of Hans Horst Meyer's classic paper in which he proposed that the ability of a substance to produce narcosis or anesthesia is governed by its partition coefficient. In this article, Robert Lipnick describes the experiments carried out by Meyer and his colleagues which disproved the earlier theories that potency was determined by the presence of particular functional groups or their metabolites or by water or fat solubility, and which led to the formulation of the lipoid theory of narcosis.