_b_. It has been generally assumed that the "A" vitamine is comparatively stable to heat. Sherman, MacLeod and Kramer state that "dry heating at a temperature of 100°C. with free access of air, only very slowly destroyed fat soluble vitamine." Osborne and Mendel reported that butter fat treated with steam for two hours and a half did not appear to have lost its value as a source of this vitamine. Drummond's earlier work with fish oils and whale oils seemed to confirm this conclusion. Sherman and his co-workers cited above put it this way: "The results thus far obtained emphasize the importance of taking full account of the time as well as the temperature of heating, and of the initial concentration of the vitamine in the food, as well as of the opportunity for previous storage of the vitamine by the test animal." More recent work by Steenbock and his co-workers in America shows that these earlier results are incorrect in the case of butter fat and that twelve hours exposure of butter fat to 100°C. may, under certain conditions, destroy the efficiency of that substance as a source of the vitamine. Drummond and other English workers have confirmed Steenbock in later experiments. Their work has shown that the presence or absence of oxygen is a factor, which may determine the extent of destruction of the vitamine. Heat alone is of very limited effect but when sources are heated in the presence of oxygen destruction of the A vitamine may be very rapid. Drummond attributes the absence of the A vitamine in lard to the oxidation that takes place in the commercial rendering of this product. We must conclude therefore that while the vitamine may be destroyed by continuous exposure to a temperature of 100°C. the effect is largely determined by the nature of the process and the way the vitamine is held in the source. Cooking of vegetables therefore will not as a rule result in appreciable destruction of this factor. _c_. The process of hydrogenation used in hardening fats appears to completely destroy the vitamine, hence the many lard substitutes now in use must in general be considered "A" vitamine-free regardless of the content of "A" in the fats from which they are derived unless they have been made by blending instead of hydrogenation. _d_. Acids and alkalies have apparently little effect on this particular vitamine. It may be well to state here however that owing to variability in behavior with variation in conditions it is dangerous to draw too general conclusions and until a given source has actually been investigated under specific cooking conditions one should not rely too strongly on analogies based on comparative experiments. This statement applies to all vitamines and presents one of the live subjects of investigation for the cooking schools and the food factories.