Forgotten mysteries in the early history of vitamin D

In the early 1920s, workers in both England and the US had discovered that rats on a rachitic diet would remain healthy if irradiated with ultraviolet light. However, they also found, to their surprise, that "control" rats to o would recover if either their jar was irradiated without the rat in it or if a cage-mate was removed for irradiation and then returned. The ideas th at either air or material objects that had been irradiated continued themse lves to convey healthful secondary radiations were investigated but not con firmed. There was then the commercially important finding that with irradia tion, some rachitic diets would become anti-rachitic. However, this effect did not explain all the previous findings. Consumption of either small irra diated fecal particles or of feces from irradiated rats was the likely expl anation for the recovery of nonirradiated rats, but this was not tested by direct experiment, and it now appears unlikely that feces from irradiated r ats would show significant antirachitic activity. It is suggested that an a lternative possibility-activity of grease from irradiated fur-deserves inve stigation.