Cod liver oil is the richest natural source of two vitamins which play an important part in the diet of both human beings and animals, vitamins A and D. That the former is readily destroyed by oxidation, through exposure to air, is generally accepted, but in spite of evidence to the contrary it is still believed by many people, particularly by those dealing with the utilization of cod liver oil, that vitamin D cannot be destroyed by exposure to air or that it is so extremely resistant to such oxidation that the question of destruction by exposure to air need be given no consideration. It seems probable that this conception of the stability of vitamin D has arisen from the fact that this vitamin was distinguished from vitamin A through a difference in the relative rate of oxidation when heated and exposed to air. As early as 1921, Mellanby1